I think there is no need to backport Firefox ESR versions newer than 78 for XP/Server 2003, as feodor2 said (more problems like inability to use single processes [multi-process are just for 2010s PCs and no one with them uses XP], no Flash Player support past v85). Even Firefox 102ESR-based web browsers (FX102 released in 2022!) doesn't have much better JS compatibility than 78 (tested on Waterfox G5, open.fm does not render correctly with it, unlike Opera GX or other chromium based browsers). I hope in future, MyPal 68 will have gradually better compatibility with 2021+ Javascript codes. So if you expected Proton UI on XP, it's unlikely to happen.