What's your computer system and Firefox version? What do you mean by disappeared? Is the browser opening/not opening? Check the computers Task Manager. If the browser is running, but not showing on the screen, it could be the window is open outside where you can see it. https://support.mozilla.org/en-US/kb/changes-toolbars-and-window-sizes-are-not-saved You may have corrupt '''xulstore.json''' file. '''Close Firefox. ''' Open your file browser to this hidden location: * '''Mac: ''' *~/Library/Application Support/Firefox/Profiles * '''Linux: ''' *~/.mozilla/firefox/Profiles * '''Windows: ''' %AppData%\Mozilla\Firefox\Profiles * *C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles * '''Windows XP: ''' C:\Documents and Settings\<user>\Application Data \Mozilla\Firefox\Profiles\ Search for the above file. Then rename or delete it. Restart Firefox.