The stock Firefox browser, with that I mean the releases that are available for download on the Mozilla website directly, can be run on 32-bit and 64-bit Windows operating systems. While that is the case, it is a 32-bit application that Mozilla offers on its website.
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.
Before we look at that, it is necessary to find out about the differences between 32-bit and 64-bit versions of Firefox, or applications in general.
Difference between 32-bit and 64-bit applications
First, it needs to be mentioned that most 32-bit applications run fine on 64-bit operating systems, but that no 64-bit application will run on a 32-bit system.
One of the advantages of 64-bit software is that it may have more memory at its disposal, provided that that enough is installed on the 64-bit operating system. The physical memory limit of 32-bit versions of Windows is 4 Gigabyte, while 64-bit versions of Windows support more than that. How much more depends on the version you are using. Windows 8 Professional 64-bit supports 512 Gigabyte for example, while Windows 7 Professional 64-bit 192 Gigabyte.
Technically, the 4 Gigabyte limit refers to the addressable memory space which the installed RAM, video cards, PCI memory range and other factors attribute to. That's the reason why you won't see 4 Gigabytes of available RAM on 32-bit versions of Windows even though you have installed that much, as part of it is used by other components. (see this for a detailed explanation)
If you are running Firefox on a 32-bit Windows operating system, it can use 3 Gigabytes of memory at the most. If you run it on a 64-bit Windows system, it can use up to 4 Gigabytes of RAM.
While it is unlikely that most users will run into the limits, new technologies such as web gaming and other real-time activities may increase the RAM needs of the browser, and may push browsers towards 64-bit eventually.
A 64-bit version of Firefox that runs on a 64-bit Windows has much larger limits, from about 8 Terabytes to up to 128 Terabytes. (see this page for more information)
It may take some time until we get there in regards to consumer PCs, but it is not that unlikely anymore that PCs have more than 4 Gigabyte of RAM installed.
Conclusion: if Firefox uses a lot of RAM on your system, you may want to consider using a 64-bit version of the browser to avoid the memory limit.
There may be other benefits: 64-bit applications can perform 64-bit register operations, which is faster than performing the same operation on a 32-bit system.
Firefox 32-bit compared to 64-bit
If you compare 32-bit versions of Firefox to their 64-bit builds on Windows, you will find out that they differ in several aspects:
- 64-bit builds are only available for the Nightly channel. They are not provided as stable, beta or aurora builds.
- You can only run "some" plug-ins on 64-bit versions of Firefox, and not all of them. While Adobe Flash and Java will work, others may not if they don't support 64-bit software.
- The crash reporter is not enabled on 64-bit versions of Firefox.
- Firefox 64-bit on Windows is not officially supported by Mozilla (which means you are on your own).
- The 64-bit version of Firefox for Windows is considered a Tier-3 platform by Mozilla, which means that it "may or may not work at any time".
You can download the latest Firefox 64-bit build on Mozilla's FTP server. This is the only -- official -- location where you can do so.
Locate the "win64" builds here and either download the zipped version, which is a portable version of Firefox that do not need to be installed, or the installer.exe version which you can install just like any other regular version of the browser.
Note: Nightly versions of Firefox are not suitable for productive environments. While they are fairly stable, you will run into issues every now and then that may break them.
If you run the browser on a system with Firefox installed, you will notice that it will share the profile with that version automatically. You cannot run both versions side by side at the same time, unless you run one of the two versions with the -no-remote -p test command (test is the name of the second Firefox profile that you want to load).
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.
Eventually, Mozilla will re-evaluate 64-bit editions of the web browser, and at one point in the future make the decision to move it to a higher build tier and offer it as a stable version next to 32-bit versions of the browser. It is not clear if that will happen in 2014, or later though.