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Firefox browser won't start.
This problem can be caused by a faulty configuration (profile), Firefox extension, a broken Firefox process or even malware. I will list some fixes that I know have worked for people.
Rename profile folder
To rename your profile folder, do the following:
1. Locate your profile folder:
C:\Documents and Settings\username\Application data\Mozilla\Firefox\Profiles
Windows Vista & 7:
2. Rename the profile in there.
3. Start firefox.
4. All your settings and bookmarks are gone, but you can copy the 'bookmark.html' from the old folder to get back the bookmarks.
If you somehow manage to start Firefox, but it sometimes still wont load, there might be a problem caused by an extension. 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.
If Firefox still wont run and there is no problem with your profile, it might be that Firefox installation is broken somehow and needs a reinstall. Download the latest version of Firefox and install. You might try to first uninstall the old Firefox. You can do so by using 'system->programs->uninstall programs'. Notice that it is found as 'Mozilla Firefox' in the software list.
Firefox affected by malware/virus
If you cant start Firefox with a clean profile, it might also be a problem that you have malware or a virus/trojan on your PC. To check for malware I have good experience with the (free) program 'Anti-Malware' from Malwarebytes. For virus scans I can recommend the (free) 'avast!' antivirus software.
It is a good idea to scan for malware/viruses every other while, because a functional virus/malware wont let you know it is there. If Firefox is broken by a virus, the virus might be incompatible with the Firefox version so you are lucky in the end :)