I had to do a double take when i read this because, as a linux user i've had a 64-bit version of firefox available since 2006ish. I figured it was windows or OSX related, but i seem to recall windows and OSX also having 64-bit versions.
The thing is they where only unofficial builds until now. So what this really means is that Mozilla is now going to be more focused on testing a 64-bit version on all OS's. Good news, except for the fact that i've already switched to chromium.