Vista (and WinXP) support ended with Firefox ESR 52.9: https://support.mozilla.org/en-US/kb/end-support-windows-xp-and-vista

Basically if you want to use a more current Firefox you need either Windows 7, 8 (8.1), 10 or to run or dual boot with a ideally pretty recent Linux distro and the EOL Windows Vista (April 2017). Windows 7 support ends in Jan 2020, Windows 8.1 is not used by many and Windows 10 is not well liked compared to 7.

Since you have a 64-bit CPU that is what many Linux distros only support now days as many have dropped support of the old 32-bit CPU's.

There are a number of light to really light options though many with a lighter desktop like XFCE should work well. You should avoid Gnome 3 and KDE if that is a rather old system with lower RAM. Say openSUSE, Xubuntu and such. Linus distros are pretty easy to use now days as much of it have GUi's to do system updates and application packages install like Firefox if you do not use the Firefox tarballs from Mozilla.