Incredibly comprehensive mod. It’s actually available directly from GOG and Steam, and it automatically installs on top of the game. You’ll hear people saying you should play vanilla if you’re playing for the first time, but no. Play Revision, just don’t enable Shift/BioMod.
Why would you want to switch? Legitimate question. 32-bit version seems to be working just fine, I doubt a music player needs the extra juice a 64-bit version provides.
These are all Firefox forks. If Firefox is done, they’re done.