“… There are 64-bit versions of Firefox, and they are made available by Mozilla for Linux distributions and Mac OS X. If you are looking for a Windows 64-bit version on the other hand, you won’t find it advertised on the official project website, and will have a hard time finding references made to it on the site as well.
“Third-party forks of Firefox exist that provide access to a 64-bit version. You can download the 64-bit version of Pale Moon for example. The browser is compatible with all 64-bit operating systems from Windows Vista on and supports all the things that Firefox supports as well.
“This may also be an option for Firefox users who do not like the new Australis interface that will be enabled in Firefox 29, as Pale Moon won’t switch to that. …”
Details Everything you need to know about 64-bit versions of Firefox for Windows