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In addition to what @dencorso said above, you may also need to enable TLS 1.0 in Internet Explorer options as the WU website no longer accepts SSL 2.0/3.0 secure connections. Internet Options > Advanced tab > Security section (scroll to the very bottom) > check Use TLS 1.0. For improved security, it's also good to uncheck Use SSL 3.0 and especially Use SSL 2.0 - unless you need to visit any specific websites not capable of anything better. I'd recommend unchecking both and turning them back on only if some site actually stops working after that.

No problem , I didn't think it was rude, it was just weird to see it written with caps as if it was supposed to be something grand when it's definitely not. I used the regular installer, not portable, but I don't know why that should make a difference here... I guess then it's back to waiting for someone to actually code something. That was really the one nice thing about the kludge: no coding required.

Please don't call it The Solution unless you put it in quotes. It's a total kludge, even if it's worked for 2 people so far. Anyway, I'm sorry it doesn't work for you. I'm curious about how it fails now, because it certainly couldn't be giving the same error message about that missing import, so is it an access violation again? I'm asking because unlike you I (and, presumably, @DanR20) didn't have any issues with some other DLL missing, so there may be something significantly different about your setup. I mean I doubt USP5.2 as such would cause this difference, given the nature of the "fix". If you set those accessibility prefs, and all DLL dependencies also work out now, and you're using v2.6 kernel, I guess we've hit a snafu with the kludge.

FYI, there's also been talk of this Firefox issue in the KernelEx thread and there's even a temporary "solution" of sorts, even if it is quite ugly. Should work for now, until someone actually gets to coding something.

Installation worked fine, but firefox.exe kept crashing on program startup. Mostly HxD.

Well, until someone develops a proper solution, there is a quick-and-very-dirty work-around that can be used. Take out your hex editor and in xul.dll, replace the string CoGetInterceptorFromTypeInfo with StgCreateDocfileOnILockBytes and CoGetInterceptor with OleGetIconOfFile. (Technically you should fix the PE checksum as well.) Basically, any not already imported function from Win2k's ole32.dll would do, I picked these because their names being the same length makes editing easy and the functions themselves seem harmless enough, should the problematic code path somehow get executed. For now at least, the added anti-accessibility precautions can probably prevent that and at least for me normal browsing worked OK. Unpredictable browser crashes are very possible, though. This was actually my first time to try out KernelEx for Win2000, and for some reason I couldn't get Firefox working with v2.8 at all (not even FF 48.0) and had to revert to v2.6. But if you had it working on v2.8 before, I'm sure it'll work with my cludge as well. Looking forward to harsh criticism from real experts for even suggesting this irresponsible half-measure

@DanR20 Since these changes seem to have to do with accessibility features, have you tried stuff like like setting accessibility.force_disabled to 1 in prefs? I'm pretty skeptical about this really accomplishing anything (ole32 doesn't seem to be delay-loaded), but I guess it doesn't hurt to try. I don't have Win2K at the ready to try this out myself.

And MS statistics of Windows 10 upgrades probably don't take into account reverters, including those who "upgraded" only for the sake of registering their free license for potential future use. I myself have added 4 machines to the "upgraded" count, none of which have actually been running Windows 10 since then.

Added the somewhat unfortunate news about Primetime CDM never getting official support on Windows XP to the top post. . @King Kadelfek: Glad I could help and I definitely appreciate you taking the time to sign up just to thank me

Did some testing with 45.2.0 ESR, and everything seems to work if an additional change is made to media.gmp-manager.url. I assume this applies to other 45 ESR versions as well. See the opening post for the details.

I've been using the Windows 7 x64 User Agent with Firefox for a long time now, upping the version numbers as appropriate. It was pretty much a necessity after our government's IT agency saw fit to persistently bombard all XP users with popups on all national and local government sites about how terribly dangerous it was to continue to use XP. (Best not to mention POSReady updates to contradict all the upgrade inevitability propaganda, huh? ) I chose the x64 UA over x86 because statistically it's the more common of the two, and to perhaps cause some confusion for any malicious scripts I might run into. Considering how often I run into those (never in my life so far), the latter is mostly an afterthought. For normal browsing, this has caused no problems whatsoever. With software downloads, the issues I've seen have had to do with "well-meaning fools" - sites that helpfully offer you downloads for your OS based on your UA, but not so helpfully don't give you an easily accessible way to download a version for a different OS. Thankfully there aren't too many such sites. In any case, even if you happened to accidentally download an x64-specific installer, it would have to be a very poorly done installer indeed if it didn't flatly refuse to install on your x86 XP system. Of all sites I have accounts at, I've only ever experienced problems with logging in to Paypal with the W7 x64 UA. I could log in OK, but for some reason never got redirected to my account page. I'm not sure if this is still an issue or not, as I've become accustomed to switching to the default UA before going there and thus haven't tried them with an overridden UA for a while. I have no idea why they'd feel the need to script their site to depend on something like that in the first place, as a text-based UI for sending money hardly seems to need any OS-specific (vs browser-specific) features.

Thanks for checking this out on Win2K! I added a note to the opening post with a link to your instructions. Also added a note about the media.gmp-eme-adobe.forcevisible pref getting replaced by media.gmp-eme-adobe.visible and media.gmp-eme-adobe.forceSupported in future versions.

That's a good point, thanks! I guess I never thought of this because if someone went out of their way to get themselves a DRM-free version of FF, they likely wouldn't want to use this plugin either - but since it's the only way known to work with XP, it does make sense that some people might still wnat to use it as an exception. I'll add this to the instructions above.

Thanks! I didn't expect anyone to pin this topic (I mean even the POSReady topic hasn't been), so since I've gotten used to skipping past the pinned topics to get to the new ones, I thought at first that it had been merged with another topic or moved. I looked around, still couldn't find it, and started thinking it had been deleted for some reason. Finally I saw it "hidden" above the ad that sort of breaks eye scanning, since the topmost pinned topic gets visually separated from the rest. Scared me there for a moment. Indeed, but I still think it's better to turn it off while testing if HTML5 video works properly. There also seem to be some sites that won't give FF HTML5 video unless Flash is turned off. I'll add it to the opening post.

TL;DR, there's now an easy way to enable MP4 (H.264 + AAC) HTML5 video support in Firefox on Windows XP using the Primetime Content Decryption Module plugin by Adobe (distributed by Mozilla). Meaning, you could get rid of Flash (with some caveats). The irony of course being that you'd have to use another thing by Adobe , but at least it's something not as prone to security problems. NOTE: If you're suddenly unable to watch videos on Facebook (or any other site) even though you haven't changed anything re: Primetime, you may have to start spoofing your browser UA (user agent) string to work around this problem. See the posts starting here for the specifics. (EDIT October 30, 2017) IMPORTANT: The Primetime CDM plugin can no longer be downloaded from Adobe servers, which means that attempts to automatically download the plugin through Firefox GUI will fail even if you edit the download configuration URL per the old instructions below. From now on, you will have to manually install the plugin for all new Firefox installations and any new profiles you create; see the new instructions below. If you already have the plugin installed, you don't need to do anything. (EDIT July 18, 2017) If you're here because MP4 video has stopped working after you upgraded to Firefox 52.0, see this post. If you still have problems, make sure you have all of the required preferences listed below and their values are set correctly. If you're here because MP4 video has stopped working after you upgraded to Firefox 49.0 or higher, that happens because 49.0 introduces a new pair of preferences. You now need to set media.gmp-eme-adobe.visible and media.gmp-eme-adobe.forceSupported to true; media.gmp-eme-adobe.forcevisible is no longer used. Toggle the Primetime plugin to Never Activate and back or restart Firefox for the pref change to take effect. NEWS July 26, 2016: Unfortunately, but maybe not all that surprisingly: Primetime CDM will not be getting official support on Windows XP after all. Says a Mozilla developer: Not to worry though, this does not appear to have any near-term (<=FF52 ESR) effect on forcing the plugin to work with the same workarounds we've been using all this time. However, just in case Adobe compiles some XP-incompatible Windows APIs into future plugin updates, you may want to save a copy of the gmp-eme-adobe folder from your FF profile or download the v.17 distribution package. (Adobe has removed the package from download but @sdfox7has kindly archived a copy (backup link).) Technically, this plugin has been added to FF for DRMed HTML5 video, but it can be used to play non-DRM as well. It's not officially supported on XP, but hey, neither is XP itself, right? I've been trying it out with FF 46 (later also confirmed with 45, 52 ESR and 47-52) on XP Pro SP3 x86 (with POSReady updates, though I doubt that matters any) for about a week now, and the experience has been pretty encouraging. The vast majority of HTML5 videos have worked without a hitch; ~5% have had 1-2 temporary freezes (the video stops, the audio continues with some crackle), which can be easily worked around by moving the video position slider back a bit and then forward again to where the freeze happened. I've seen only one (1) case of a show-stopper error message about "corruption or unsupported features in video" (quote not verbatim) that couldn't be worked around by jiggling the slider (not a crash, just an error message and the video stopped). You may not necessarily be as lucky, though, as the reason the plugin is not included by default by Mozilla is its reported crashing on some high profile sites. (If you're interested in getting this to work on Windows 2000, be sure to read @blackwingcat's post below in addition to this one, and why not his blog posts (1, 2, 3) as well (in Japanese, but with screen shot images).) To be able to install and operate the Primetime CDM, add and/or set the following FF preferences in about:config (be careful, all of these are cAsE sEnSiTiVe!): media.gmp-eme-adobe.enabled (boolean; true) media.gmp-eme-adobe.forceSupported (boolean; true; FF 49.0+) media.gmp-eme-adobe.forcevisible (boolean; true; FF 45-48, no longer used in 49.0+) media.gmp-eme-adobe.visible (boolean; true; FF 49.0+) media.gmp-eme-adobe.version (string, not integer; 17; required for manual install now that automatic install is no longer an option) media.gmp-eme-adobe.abi (string; x86-msvc-x86 (EDIT: x86-msvc-x64 for 64-bit XP users); not required, you can set it if you want to more closely mimic automatic install) media.gmp-eme-adobe.lastUpdate (integer; 1500000000; not required, you can set it if you want to more closely mimic automatic install) media.gmp.decoder.enabled (boolean; true) media.eme.enabled (boolean; true; defaults to false if you downloaded a DRM-free version of FF; thanks @heinoganda for pointing this out) media.gmp-provider.enabled (boolean; true) may also be needed, if not already present, as found out by @VistaLover, in the case of SeaMonkey 2.49.5... To manually install the actual Primetime plugin software, first download the ZIP package (backup link) from @sdfox7's site (he has plenty of other useful XP stuff there as well, definitely worth checking out!). You can use the following checksums to verify package integrity: To install the plugin: Open your FF profile folder. An easy way to navigate to it is by opening about:support in FF and clicking the Open Folder button next to Profile Folder. Inside the profile folder, create subfolder gmp-eme-adobe and open it. Inside gmp-eme-adobe, create subfolder 17 and open it. Unpack the Primetime ZIP archive into folder 17. Folder [your FF profile path]\gmp-eme-adobe\17 should now contain three files: eme-adobe.dll, eme-adobe.info, eme-adobe.voucher. After this, check the Plugins in your Add-ons list and you should see the Primetime plugin listed and ready for use. If you don't see the plugin there, make sure you've set all of the required preferences as described above and put the files into the correct subfolder. Restarting Firefox to complete the manual install isn't usually required, but try it if the plugin doesn't appear, (Also, remember that if you have more than one Firefox profile, you'll need to repeat these steps for each of them individually.) Set the Primetime plugin to Always Activate, Flash to Never Activate (not strictly necessary, but better for testing), and go to Youtube HTML5 video player check page. Both H.264 and MSE & H.264 should have blue checkmarks. You may want to try this test page (kindly suggested by @dencorso) or some Twitter videos to quickly make sure H.264 really is working (with Youtube you can get fooled by WebM getting played instead). In theory, you could uninstall Flash at this point, but realistically it's probably wiser for now to leave it installed and disabled in the browser. Personally, I installed the FlashDisable extension to quickly toggle Flash on some of the sites I frequent that don't support HTML5 video. Some of those insistently serve Flash video to desktop FF without even checking if it can do HTML5 video or not. That can be worked around by faking a different user agent string, but for now I've found it more convenient to just temporarily toggle Flash back on for them. FlashDisable supports the Ask to Activate setting, so you don't have to worry about other sites if you forget to turn Flash off again. Based on what Mozilla devs have been saying on Bugzilla (Unhide Adobe GMP on Windows XP, Hide Adobe GMP on Windows XP in Firefox 46 and 47, Make Adobe GMP available to Windows XP users in Firefox 45 and later) it seems quite possible that this plugin could soon (FF 48+) be made available without having to use any tricks (or pre-release versions), but I suppose it's also possible that the current situation will continue indefinitely (or even that the plugin will eventually be made more difficult to access under XP). (EDIT July 18, 2017) As you've likely heard, FF 52 ESR is the last version of the browser that's going to work on XP (unless some intrepid person forks a special version for us). In a way, this is a blessing in disguise, as Primetime support has been completely excised from FF 53 and Google's Widevine CDM doesn't have our helpful side-effect (and doesn't work on XP), but ESR 52 will be supported well into 2018. So, why not make use of it while we can, right? Note that HLS streaming is not natively supported with this plugin. It does work on sites coded to use some JS library or player to work with HLS, but not for directly playing .m3u8 video sources. You may want to keep an eye on developments with hls.js in general and firefox-hls in particular if a favorite site of yours falls into the latter category.