The stable, supported, mainstream version of Firefox on Windows is a 32-bit application. Even if you use 64-bit Windows, if you use Firefox, you’re using a 32-bit browser. The exception is if you’re using the Nightly build of Firefox. This represents the latest, cutting-edge version of the browser, and it’s available in two versions: a 32-bit one, and a 64-bit one.
However, this won’t last much longer. Mozilla announced last week it was no longer going to produce 64-bit Nightly builds of Firefox for Windows; nor will it run automated tests of 64-bit Firefox. The browser’s future on Windows is resolutely 32-bit. Linux and Mac OS X, in contrast, both have official 64-bit versions.
Several reasons were given for this discontinuation: many plugins have no 64-bit version, Mozilla’s bug reporting and tracking infrastructure provides no clear distinction between 32-bit and 64-bit problems, bugs go unfixed because the 64-bit Windows version is not deemed a priority, and JavaScript performance in the 64-bit builds is substantially slower than in the 32-bit version. Further, Mozilla developers say they won’t fix any bugs that only manifest in 64-bit versions. Firefox developers say a fully supported 64-bit version of Firefox won’t be released in the first half of 2013 and it probably won’t make the second half either.
This is an unfortunate decision by Mozilla. 64-bit support is not important to all applications, but when it comes to Web browsers, it has a valuable role to play. At its core, the difference between 32- and 64-bit comes down to memory. Each 32-bit process gets 232 bytes (that is, 4GB, of memory to use). Each 64-bit process gets 264 bytes, 18EB, four billion times more memory, to play with. Of that 4GB that 32-bit processes can use, there’s a further limitation: 32-bit Windows reserves 2GB or 1GB for itself, giving each 32-bit application only 2 or 3GB to use. 2GB is the default; to get 3GB, the application needs to opt in, and Windows needs to be booted in a special mode. On 64-bit Windows, 32-bit applications that opt in to 3GB support actually get almost the entire 4GB to themselves.
Full Story: 64-bit Firefox for Windows should be prioritized, not suspended | Ars Technica.