Well they’ve been talking about this for a while (a year or two?) so naturally the solution was found a while ago too. Originally there wasn’t a need for any patches, kernel mods or source code modifications (Firefox is after all open source) to make FF53 nightly work on XP (only change was in the installer), not sure if that’s changed now but in any case it will not stop committed XP users from using it.
Remember how Mozilla (equally screaming and shouting in public announcements) ended Firefox support for Windows 2000 many years ago, saying Firefox 12 was the last version to be supported? Here is Firefox 53 running in Windows 2000.
But truth be told, I (as an user of Win2k/XP exclusively) am still on FF28 and see no reason at this point to upgrade. If Firefox 29’s interface changes weren’t bad enough (although I can deal with them by modifying my style script) when they began messing with the browser fonts I gave up on even bothering to modify the newer versions to fit my needs. Maybe that can be fixed but I’ll pretty near have to do it in the source code, and the cost-benefit analysis of doing that work over sticking with FF28 is very decisively in favor of the latter.