In Vista Ultimate (64-bit here, and probably on 32-bit as well), the path is:
Drive letter:UsersusernameAppDataRoamingMozillaFirefoxProfilesxxxxxxxx.default
That doesn't seem to work, at least not for me on this machine. Is Vista that different, or did I do something wrong? I pasted the user.js file into that folder, but still the problem exists.

However, CTRL+X, CTRL+C and CTRL+V still work fine, and is what I've gotten used to anyway.
So... are we any closer to a .js that enables FF's spellchecker in 95,98,NT,2K,XP, and Vista?