Dave-H

I agree as a general off-topic observation that while you do, of course, need to have a general awareness of security and privacy issues when working online, there's really no need to be paranoid about it.

We all know where this is going to go. Please get back on topic.

Edge appears to be the same, so I guess it's a Chromium thing?

Back on topic please guys.

Thanks very much for the explanation!

Thanks @roytam1. Yes, I'm aware that the runtime version I'm using isn't an official XP-compatible version. It was brought to my attention by @Monroe in another thread here. I think it was developed unofficially to help with some games which would not otherwise work on XP. Presumably, as I've now found, it can also cause compatibility problems with some other programs! I will probably keep it installed as a system runtime, but be aware that if any other program now fails to run, that might be the cause. Hopefully all I will need to do then, as with Firefox, is put compatible versions of the necessary runtime DLLs in the program's folder.

OK, it's fixed! The portable version worked fine, and in fact if I ran the firefox.exe in the portable version's program folder, rather than firefoxportable.exe, it opened fine using my normal Firefox profile! So, I started comparing the two installations, and found that some DLLs in my Firefox program folder appeared to have been disabled by renaming 'DLL' to 'D~L'. That's my normal way of disabling a file, changing the middle character of the extension to '~'. Why I had done that is lost in the mists of time in this case, but one disabled DLL file stood out, and that was msvcp140.dll, which is a C++ Runtime file of course. That reminded me that I had recently updated my C++ 2015-2019 Runtime to 14.29.30139, following a recommendation elsewhere on MSFN. Thinking perhaps this could be the issue, I simply renamed the msvcp140.dll file in the Firefox folder back to normal, and everything came good! So, I guess the new system runtime file versions may not actually be compatible with all programs, so I will now be on the lookout for any other seldom-used programs which no longer work. Thanks everyone for the help as always! Cheers, Dave.

No, Firefox was not open when I uninstalled Malwarebytes. I hardly ever use it now. Thanks for the link to the portable version, I will give it a try and report back.

OK, I tried disabling the current Firefox profile (by renaming its folder and making a new empty folder with the same name). Firefox still didn't start, with the same immediate error. I think that means the issue is nothing to do with anything in the profile.

There is no noticeable increase in startup time with 22.03.05 on my machine, it seems perfectly normal as it was before Panda was installed, so I can only assume that it just doesn't run well on your particular hardware.

I did wonder about extensions, I haven't changed those on FF 52 for years, but they do update themselves so it's possible that an incompatibility has crept in. It won't run in Firefox 'safe mode' either though, which I thought was supposed to not load any extensions or plugins. I'll try with a new clean profile which should eliminate this possibility.

Here's what Dependency Walker says. CreateThreadpoolWork is indeed showing an error, but so are several other functions, and they're not stopping it from running! Incidentally, why does it say 'Not Bound'? Should that actually be 'Not Found' or is that terminology correct? Sorry, I'm no expert on this sort of thing!

Thanks everyone, and sorry for the delay in replying. I was having terrible problems accessing the site yesterday, which i hope is now fixed. @NotHereToPlayGames I ended up uninstalling Malwarebytes in Safe Mode because when I tried in normal mode, when I said yes to 'do you want to uninstall' the machine immediately rebooted! Rather than try again and probably have the same thing happen, I then tried in Safe Mode, and it immediately uninstalled, very quickly indeed. Why the machine rebooted, I have no idea. I doubt this has anything to do with Firefox now failing to run, but you never know! @schwups @NotHereToPlayGames I'm not running any form of extended kernel. @ED_Sln Firefox.exe looks to be OK. Very strange. Just to add that Firefox won't run in its own 'safe mode' either.

I do have Revo installer, if you remember I used it to give AstroSkipper a list of .NET updates in another thread. I don't think I've ever actually used it to uninstall anything though! If I did, it would have been a very long time ago, so not related to this issue I'm sure. I have recently uninstalled Malwarebytes 3.5.1 though, using the normal uninstallation routine in Safe Mode. I wonder if there's any connection there?

PortableRegistrator 1.5 certainly works with XP, I don't know about any later versions. I can upload it for you if you can't get a version which works. FWIW I use the Classic Blue theme, which always looks very pleasing to me, but it may not help with your issue of course.