Not knowing any details but multiprocess just means it can run multiple copies of firefox.exe out of one window, like chrome. If it is good or bad depends how they implement it. I ran chromium snapshots as my main browser for over a year. It got to the point I had to use a command line switch to limit the number of chrome.exe instances. If I had 6 tabs open it was running at least a dozen copies of chrome.exe at one point.
That's when I tried FF again after some time away. When it got fast using only one copy of firefox.exe that's when I went back to it.
I can imagine multiple copies of 64 bit Firefox in Windows could eat up huge chunks of memory. But the proof will be in the trying. I'll probably try it on a Windows 7 x64 VM first.
Edit: The old becomes new again. Unix type systems used fork() to create another copy of the running application. That became passe when threads became the rage. Now with huge address spaces available I guess there's no longer any need to be stingy with resources.
Edit2: as for the 64 bit, the irony is on Windows if you have 4 GB ram or less the 32 bit version will probably be faster. At least the latest FF 32 bit installed I'm using now, 33.1 feels as fast as any x64 flavor I've tried. 8 GB or more may be a different story though.
But this is a Laptop with a Sata II spinner. Not super fast in any case. But one thing I learned with my old 486, sometimes it's easier to feel the performance differences on a slow machine. Slow code felt really really slow.