Sorry to hear about this problem. It's possible that Firefox is stuck in memory and you need to terminate that non-working process before you can restart it again. One way to do that is shut down and restart Windows. But you also could try the Windows 7 Task Manager.

To call up the Task Manager, you can press Ctrl+Shift+Esc. Normally the first thing you see is the Applications/Tasks tab, which should list all the visible windows. If you see Firefox here, try using "Switch to" to access it. However, I suspect you won't see it here.

Click the Processes tab and look for firefox.exe and then right-click > End Process to terminate it. This might take several seconds to complete. Then try starting Firefox in Safe Mode again (holding down the Shift key).

Any luck?

firefox.exe does not appear in the Processes tab

Thanks for checking that.

Did you just update recently? Sometimes during that process the program files are damaged. Please try this:

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent, corrupted, or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

(A) Download a fresh installer for Firefox 40.0 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(B) Exit out of Firefox (if applicable).

(C) Rename the program folder, either:

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\OldFirefox

(D) Run the installer you downloaded in #1. It should automatically connect to your existing settings.

Any improvement after this reinstall?

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins