Further, Mozilla developers say they won't fix any bugs that only manifest in 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. Linux and Mac OS X, in contrast, both have official 64-bit versions. The browser's future on Windows is resolutely 32-bit. 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. 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. The exception is if you're using the Nightly build of Firefox. Even if you use 64-bit Windows, if you use Firefox, you're using a 32-bit browser.
The stable, supported, mainstream version of Firefox on Windows is a 32-bit application.