DailyTech reports the next version of Firefox may get an official 64-bit build. Unofficial 64-bit builds of Firefox are already available over here, but the disadvantage of these versions is that they aren't compatible with Mozilla's plug-in system.
Advantages of the transition to 64-bit include faster speeds on pages using encryption, fast-call type function calls, and more efficient memory mapping of large files. Initial benchmarks from the 64-bit build show impressive gains in certain applications.
A notable downside of the patch for Firefox 3.5 is that it breaks Mozilla's plug-in system, which relies on a 32-bit architecture. However, Mozilla has talked recently of adding official 64-bit support in the near future, so this will likely be addressed shortly.