Most users will want Flash Player on their system. As much as I hate Flash, their will still be users who will need it for various websites, at least until those sites switch to HTML5. Also, Netflix users will want to have a working Silverlight installation. If x64 Firefox doesn’t work with these two plugins, it’s value will be greatly diminished.
As for everyone else running x64 versions of Windows, this new version should provide for a slightly faster web experience.
PS: I could be wrong, but I think that versions of Firefox for GNU/Linux need to be compiled for whatever processor they will be running on. Therefore, x64 users can expect x64 versions of Firefox to already be installed, although one could theoretically run x32 versions if they have x32 wrappers. If I’m right, us x64 GNU/Linux users have been running x64 Firefox for years.