In Firefox, type
about:support in the the URL window. If you have the menu display turned on, this is also accessible under
Help | Troubleshooting Information. The
Application Basics section at the top will show you the version you're running.
Now that the 64-bit version has been officially released, it carries the same version number as the 32-bit version. So if the version numbers on your computer are the same, you can determine 32-bit vs. 64-bit from the User Agent entry in that section:
- 32-bit Firefox running on 64-bit Windows (8.1, can't verify 10), shows WOW64 in the User Agent.
- 64-bit Firefox running on 64-bit Windows (8.1, can't verify 10), shows Win64; x64.
- 32-bit Firefox running on 32-bit Windows will not include the Win64; x64 in the user agent.
Another place you can differentiate them is the
Control Panel | Programs and Features | Uninstall a program. The software name is listed (the version number, 43.0 in this example, may be different):
- 32-bit Firefox is listed as Mozilla Firefox 43.0 (x86 en-US).
- 64-bit Firefox is listed as Mozilla Firefox 43.0 (x64 en-US).
If you have something other than the US English version, the letters after the x86 or x64 will be different. (Source: How to tell if Firefox is 32-bit or 64-bit)
One other place to check, according to Caspid on the Mozilla Support forum, is the Task Manager.
- Start the Task Manager (Ctrl+Shift+Esc).
- 32-bit Firefox will display as
user559862 reported (in a now deleted post), that 64 bit Firefox is displayed as simply firefox.exe in the Task Manager.
Build Platform Target
It looks like there's yet another place to identify this, which you found, and I'll add it here for completeness. If you enter the URL
about:buildconfig (which also has a link in the Application Basics section), it has a section called
Build Platform Target. This includes a similar identification indicator. A 64-bit Firefox will include the identifier