Mozilla published the Windows 64-bit installer for Firefox to the Download page with yesterday’s release of Firefox 43.0. You must have a 64-bit version of Windows to take advantage of the new version of the browser.
There are two cosmetic bugs with the 64-bit version of Firefox that have been posted for a while without resolution. The first bug addresses installing the 64-bit bit version of Firefox when you have an existing 32-bit version installed. The installer will upgrade Firefox to the 64-bit version but install it to C:Program Files (x86), breaking Windows conventions of installing 64-bit applications to C:Program Files. The second bug is asking for a designation of which bit browser you are using in Firefox’s Help, About.
Since both of those bugs make it difficult to tell which version of the browser you are using, here are two different ways to identify 32-bit vs 64-bit Firefox.
In Firefox type about:support into the address bar. This will open up a page with a lot of information about your browser. From the User Agent line under Application Basics, you can see if this is the 32-bit version or 64-bit version running.
The 32-bit Firefox running on a 64-bit Windows 8.1 shows WOW64 in the User Agent.
The 64-bit Firefox running on a 64-bit Windows 8.1 shows Win64; x64.
The 32-bit Firefox running on a 32-bit Windows will not include the Win64; x64 in the user agent.
Programs and Features
Another place that you can check is the Control Panel, Programs and Features. To uninstall a program, you can see the software name listed.
The 32-bit Firefox is listed as Mozilla Firefox 43.0 (x86 en-US).
The 64-bit Firefox is listed as Mozilla Firefox 43.0 (x64 en-US).