Summary: The Firefox update incorrectly installed the 64-bit version of the program even though I always used the 32-bit version (although I have a 64-bit Windows 7).
Symptoms:
- Firefox does not start, not even in safe mode, not even "profile manager"
- Deleting or moving profile folder and profile.ini has no effect
- If you start "Windows Task Manager" you will see firefox.exe in the processes tab for about 4 seconds every time you try to start it
- In the Task Manager it does not say "*32" after firefox.exe (which indicates a 32-bitt app)
- Your installation files are in "C:\Program Files (x86)" (not "C:\Program Files" as they would be if a 64-bit install). (Note: Exact path name depends on Windows language, but copy-pasting it works.)
- In Windows Event Viewer (right click "My Computer" then "Manage", "Event viewer", "Custom views", "Administrative Events") you will se an error logged for each start attempt, with error code 0xc0000005, Process-ID 0xcc4 etc.
- In the list of installed programs (Windows Control Panel / Programs...") for Mozilla Firefox it says "x64" for 64-bit, but it should say "x86" for 32-bit.
- No crash reports
Download the latest (56.0.1) 32-bit installer and run it with default options. It will recognize that you already have an install and assume you try to upgrade. The final stage is a button not named "install" but "upgrade".
Safety measures:
Obviously, running a new installer has the risk of overwriting present data. For me it worked fine, it started up perfectly with all bookmarks, plugins etc working. But to be on the safe side, before trying my fix I recommend to do all of this:
- Make safety copy of profiles folder and profiles.ini, which are located in %appdata/Mozilla
- Make safety copy of the entire installation at C:\Program Files (x86)\Mozilla Firefox
- Most importantly: Create a "restore point" in Windows System Restore (by right clicking My computer, choose "properties" then System Protection), this should give you an easy way to get back to your previous state if you want to (I know some don't believe much in restore points but they work for me 99% of the time)