I'd personally recommend the K-Meleon Goanna builds as the most likely to work with XP/Vista since they use native Win32 stuff for the UX, unlike both Pale Moon and Basilisk. I also want to see ChatZilla and some other plugins be ported to KM
That said, I'm sad to see 52 ESR go away. It will mean a lot of the less technically inclined XP/Vista users now will throw out their old hardware after it goes EOL.
All distros of Linux except Debian 9 LXDE runs absolutely appallingly on my netbook, so it's XP or nothing.