uilds of Firefox have been available since before Firefox was Firefox - as a matter of fact, the Mozilla suite was available as a 64-bit compiled binary on Alpha chips and more running X11 window servers - such as, Solaris, HP-UX, Linux etc.
The first 64-bit optimized Firefox version however was version 3.0, following the general cleanup done in the "reflow" branch that resulted in Firefox 3.0 using Cairo as a rendering layer.
Since 2003, a nice little plugin named nspluginwrapper allows 64-bit users to run 32-bit plugins in 64-bit Firefox: using a very specialized and tweaked fork of Wine, it is perfectly possible ever since Firefox 1.5 to run 32-bit Flash and 32-bit Adobe Reader plugins in 64-bit Firefox. Stability isn't really great, but with the fall of Adobe Reader, Java 1.6's 64-bit browser plugin and the Square test builds of Flash, running a fully 64-bit Firefox is possible in Linux.
Another small advantage that appeared in Firefox 3.0 was the ability to make use of basic hardware accceleration through XRENDER; ever since Firefox 3.0, Linux users were able to enjoy hardware-accelerated (granted, not as accelerated as Firefox 5.0 on Windows currently is, but still), fully 64-bit browsing.
And compared to the same install on 32-bit Windows, it really could scream.