Mozilla has begun offering highly experimental builds of a 64-bit version of Firefox.

Given that Mac OS X, Windows 7 and most versions of Linux are all available in 64-bit, it makes sense that Firefox joins browsers like Safari and Internet Explorer in offering 64-bit builds.

Fans of 64-bit computing might know that unofficial 64-bit builds of Firefox have been around for some time, and many 64-bit Linux distros also offer 64-bit builds of Firefox. However, these packages (which were posted over the long weekend) are the first official releases sanctioned by Mozilla.

For now, the new 64-bit Firefox is for Windows machines only. Mozilla is planning to make official 64-bit builds available for multiple operating systems when Firefox 4 is released later this year. However, the company says that at this point, these builds are just for experimentation and testing.

You will only be able to run a select few add-ons in 64-bit mode, as there are only a handful available. The biggest downer is that there’s no 64-bit version of Flash Player for Windows 7. There is a beta version of 64-bit Flash available for Linux that you could run with one of those aforementioned Linux builds. Though having tested it on Fedora, we would not recommend it for general use.

There’s no installer for Firefox 64-bit, and it also requires that you install the Microsoft Visual C++ 2010 Redistributable Package (x64) and quite a few other workarounds. In other words, not for the faint of heart.

There’s some debate as to how much benefit users will see from a 64-bit web browser. Since the main appeal of 64-bit apps is the ability to use huge amounts of RAM, it makes sense for giant desktop workhorses like Photoshop to be 64-bit. Browsers are, by comparison, much more light-duty. You could certainly devote a lot more memory to Firefox if you’re running 64-bit, but even if you keep dozens of browser tabs open and run several web apps at once, it’s unlikely you’ll need more than a few hundred megabytes of RAM.

Then there are add-ons. Most add-ons will work, but they will run in 32-bit mode, at least partially negating the benefit of having a 64-bit browser. Add-on makers will eventually catch up and rewrite their code for the 64-bit browser, but it will take years to get there.

While there are questionable gains from having a 64-bit browser, Mozilla isn’t alone in building one. Safari on OS X 10.6 is 64-bit, as is IE8 on Windows 7. Google is also hard at work on a 64-bit version of Chrome.

Unfortunately, since Adobe has not yet released an official 64-bit version of Flash Player, all of these next-gen browsers still need to run at least some code in 32-bit mode.

For more info on the 64-bit version of Firefox for Windows and how to get it running, check out Firefox programmer Armen Gasparnian’s announcement, which also has installation instructions.

See also:

• First Look: Firefox 4 Preview Delivers Speed, Revamped Interface
• Firefox Has Been Hitting the Gym — Version 3.6 Is Faster, More Capable