wont-open.com - for all things that won't open!
Firefox won't start.
Rename profile folder
Go to your profile folder:
C:\Documents and Settings\username\Application data\Mozilla\Firefox\Profiles
Windows Vista & 7:
and rename the profile in there. Start firefox. All your settings and bookmarks are gone. You can copy the bookmark.html from the old folder to get back the bookmarks.
There might be a problem caused by an extensions.
Use Tools->Addons->Extensions to disable all extensions.
Firefox is already running (process running)
The firefox window might be closed, but the process could still be active. On windows use Task-manager (Cmd-Alt-Del) and locate the Firefox process and close it.
Firefox is already running (lock file)
It can be that the PC thinks firefox is still running, because a lock file was not released the last time firefox was closed.
Go to your firefox profile folder and delete the 'parent.lock' ('lock' in linux) file.