64 bit builds

From MozillaZine Knowledge Base

You can run a 32 bit version of a Mozilla application using a 64 bit kernel (Windows 7 x64 etc.) though a 64 bit version "supposedly" increases performance. However, unless the the application is able to take advantage of the wider registers (typically multimedia encoding/decoding, cryptographic, or number crunching applications), you may not see any performance improvement. If you had poor performance before you might not notice any improvement. Typically you only want to use a 64 bit version if the application exceeds the 4GB virtual memory space limit.

This updated blog post benchmarks 32 and 64 bit versions of Firefox, including Waterfox (specifically compiled for 64-bit) and concludes that "We should see some performance improvements in the future, but Firefox 64-bit doesn’t appear to give you a faster browsing experience at this time." This Ghacks article compares 32 and 64 bit versions of popular browsers and concludes "Most differences are marginal and not visible to the user. "

  • Code that uses a lot of pointers will consume more memory. 64 bit pointers are larger than 32 bit pointers, so there's more memory to move around, causing a potential performance hit. The effective processor cache size will also be reduced.
  • Code that is not careful about memory packing/alignment can run less efficiently as 64 bit.
  • Code that uses large data types (64/128 bit WORDs) will be faster because you don't need to play the 'split it into 32 bit WORDs' game.
  • A 64 bit version might be faster due to it being optimized for CPU's that support x86-64 (it has twice as many general purpose registers and SSE/SSE2 registers as a x86 CPU, and they're all twice the width).



64 bit builds for Linux are available at . Many Linux distros provide their own 64 bit builds of Firefox, Thunderbird, and SeaMonkey in their repositories.


64 bit builds for Mac OS X are available at .


Official 64 bit builds of Firefox for Windows are available on the normal download page starting with version 43.0. There aren't any official 64 bit builds of Thunderbird and SeaMonkey for Windows. [1]

64 bit release builds for Thunderbird are available from some Mozilla web sites. They seem to work fine but are not tested and are unofficial/unsupported. The reason why they're marked as release builds is due to changes in the build system. They won't be officially supported until you see a statement to that effect in either a official blog post or the release notes, or you can download the setup program from the web site. There is no ETA for when they will be officially supported. What unsupported really means is that QA doesn't test it and you're more likely to run into a regression bug than a supported build. It doesn't mean that you can't get help for it in the forums. [2]

If you do run into a problem (that you want to report as a bug or ask for help using the forums) its recommended that you use custom setup to install a 32 bit build in a different directory, and try to duplicate the problem with the officially supported build. The two builds will use the same profile and have separate shortcuts.

Forcing a automated update (using Help -> About Thunderbird or Tools -> Options -> Advanced -> Update -> Check for updates) works for 64 bit release builds. This was confirmed by the update from 60.0 to 60.2.1.

Tracking bugs:

Windows x64
Windows 64 bit builds in Thunderbird
64 bit Windows package for Lightning

Third Party Builds

WaterFox and Pale Moon are optimized 64 bit versions of Firefox for Windows. Start64 and Firefox 64 have several 64 bit builds of Firefox (nightly, WaterFox etc.) for several operating systems.[3] .

The Third Party/Unofficial Builds forum has optimized 3rd party builds that may include some 64 bit builds for Windows.

Extensions and themes

Most extensions and themes don't care what operating system you are using, or whether you are using a 32 bit or 64 bit application.

You can not use a 32 bit build of Lightning with a 64 bit build of Thunderbird or SeaMonkey. There are official 64 bit builds of Lightning available for Linux and OS X but they're for nightly builds. Nightly builds are automated untested builds . Unofficial 64bit builds of Lightning for Linux are available at here. Also look in the contrib subdirectory of the latest release in


You need to use a 64 bit version of a plug-in if you use a 64 bit Firefox build. The Adobe and Foxit web sites auto-detect whether you use 32 or 64 bit.