Firefox stops working. No error messages. No crashes. It just stops. Also, reinstalling doesn't fix it.

Here's how I got out of that situation:
* Uninstall Firefox.
* Go into C:\Program Files\ and move the Mozilla Firefox folder to the desktop.
* Restart the computer
* Move the Mozilla Firefox folder to the trash, and empty the trash.

At this point, most folks can reinstall, but if you used FrontMotion Firefox (a Firefox installer that comes as an MSI file), you will have to go into the registry, search for FrontMotion, and then delete the CLSID for that version of Firefox). More details below.

* Reinstall.
* Firefox should start up fine.

More info at MozillaZine.

Using Regedit to Alter the Registry

Uninstalling the old FrontMotion Firefox seemed to leave behind a registry key that may be impeding a new install of Firefox. So you have to remove that object.

Windows-R
regedit
Press F3
Search for "FrontMotion" or whatever you think might work.
You should look for the string that's inside a CLSID object.
Delete that object.

Also published at eHow.