32bit executables can use 4GB address space on windows if they have the LARGEADDRESSAWARE flag set, which has been the case for firefox.exe since 2010[1].<p>It may appear that they use less than 4GB if you look at the working set or private bytes figures instead of virtual size because they reserve more address space than they allocate. Or it might crash before reaching that limit due to large allocations failing.<p>But 64bit builds have been available for years on their ftp server, they just weren&#x27;t supported. But since last december they are[2], so there really shouldn&#x27;t be any need to run 32bit builds on a 64bit system.<p>[1] <a href="https:&#x2F;&#x2F;bugzilla.mozilla.org&#x2F;show_bug.cgi?id=556382#c34" rel="nofollow">https:&#x2F;&#x2F;bugzilla.mozilla.org&#x2F;show_bug.cgi?id=556382#c34</a>
[2] <a href="https:&#x2F;&#x2F;blog.mozilla.org&#x2F;futurereleases&#x2F;2015&#x2F;12&#x2F;15&#x2F;firefox-64-bit-for-windows-available&#x2F;" rel="nofollow">https:&#x2F;&#x2F;blog.mozilla.org&#x2F;futurereleases&#x2F;2015&#x2F;12&#x2F;15&#x2F;firefox-6...</a>