This article describes what to do if Firefox doesn't open when you click the Firefox icon.
If you get the "Mozilla Crash Reporter" window when you start Firefox, see Firefox crashes - Troubleshoot, prevent and get help fixing crashes for solutions.
See How to run Firefox when your profile is missing or inaccessible for solutions.
This error is caused by a problem with Firefox program files. The solution is to remove the Firefox program and then reinstall Firefox. (This will not remove your passwords, bookmarks or other user data and settings which are stored in a separate profiles folder.)
If you see the message, "XULRunner - Error: Platform version <x.x.x.x> is not compatible with minVersion <x.x.x.x>"" ("<x.x.x.x>" depends on version number), after a software update, use another browser to download the Firefox installer from the Firefox download page, save it to the desktop or other location, delete the Firefox installation directory and then reinstall Firefoxreinstall Firefoxreinstall Firefox.
This error indicates that either Firefox is not installed or your shortcut to it is incorrect. Make sure that Firefox is installed at the location specified by the shortcut. If it isn't, install Firefox if necessary, then modify the shortcut to point to the correct location.
If you receive the above error when launching Firefox on a Windows system, you may be required by Firefox to log in as an administrator. You can either reboot the computer or, if you have an administrator account, you can execute the program as an administrator.
If you get another error message (e.g. "XML Parsing Error: undefined entity Location", "Secure Connection Failed … uses an invalid security certificate"), see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.
If you click the Firefox icon but no Firefox window opens and no error message is shown, a simple first step is to restart your computer and then try again. If Firefox still does not start, see the following sections.
If Firefox previously worked but stopped after you installed extensions or themes, see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.
If Firefox previously worked but suddenly doesn't start, it may be due to some corrupt data in your settings. Uninstalling and reinstalling Firefox would not fix this problem because your settings are not removed on uninstall.
To test to see if this is the problem, use the Profile Manager to create a new profile. If the problem goes away, you can then move your bookmarks and other important settings into the new profile.
If you installed Firefox outside of your package manager, make sure that your system has the libraries listed in the system requirements.
Based on information from Browser will not start up (mozillaZine KB)