The problem may be caused by an extension; see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and try starting Firefox in Diagnose Firefox issues using Troubleshoot Mode.

The process may be hung, so you may need to end it before trying to troubleshoot with safe mode. To see if this is the case, follow these steps:

1. Press CTRL+SHIFT+ESC to load the Task Manager window
2. Click the Processes tab at the top. (Click once near the top of the window if you don't see tabs)
3. Find all firefox.exe processes listed below
4. On each firefox.exe process, right click and choose __End Process
5. Try starting Firefox again (in safe mode)

If it won't start in safe mode even after ending the process, the next step is to try removing Firefox's installation folder in C:\Program Files. This won't affect any bookmarks or settings, as these are stored elsewhere in the profile folder.

Thanks. I tried this. Not even safe mode will launch. Now I must consider a windows wipe. Suspect a virus.

Did you try the clean install? To do this, you must complete remove or rename the Mozilla Firefox folder in Program Files. (Uninstalling isn't enough)