Warning to users of Adobe's "Flash" plug-in: I have discovered that the 52 ESR version of the UOC Patch sets two preferences that interfere with Flash:
privacy.resistFingerprinting
dom.ipc.plugins.asyncdrawing.enabled
#1 is more serious. For some reason, it prevents Flash from working at all. Luckily, it also has the easiest workaround: set it back to false, or remove the line enabling it from the UOC_Patch_52.js file; instead, use an extension (I use "Canvas Defender" but there are several others) to resist canvas fingerprinting.
#2 may, or may not, affect your system. To test, first disable the above preference, then go to https://get.adobe.com/flashplayer/about/. If both the Flash animation and your currently installed Flash version appear, you're fine and can leave this preference set to true. If the page doesn't work properly, you'll have to set this preference to false also. Unfortunately I know of no workaround to retain its function if you have to turn it off.
Edit: It turns out that privacy.resistFingerprinting implicitly does lots of things "behind the scenes". Some of these changes, such as user agent overrides, could cause Web sites to malfunction or otherwise affect usability. Luckily, most of the changes didn't land until FF 55 (which won't run on XP) or later FF versions (but Serpent 55 may include some of them). Still, I think it would be best to remove this preference from the UOC Patch.
Edit 2: The troublesome privacy.resistFingerprinting also appears in the FF 45 version of the UOC Patch, but that preference doesn't do as much in that version, so it may not cause trouble with Flash there. I haven't tested FF 45 with the UOC Patch and Flash.
Edit 3: Found one more. This one's more obscure, but I'm posting it anyway, just in case anyone runs across it. The UOC Patch sets pref apz.allow_zooming, which conflicts with the Multi-Account Containers add-on. That add-on is the main reason I switched to Serpent 55, so my own customized version of the UOC Patch now omits privacy.resistFingerprinting and apz.allow_zooming, and sets dom.ipc.plugins.asyncdrawing.enabled to false to ensure Flash always works.
Hopefully I now have everything working again!