I had the same issue not so long ago. I could not reset the browser, or even create a new profile. I ended up doing this; '''Close Firefox'''. Open the file browser / explorer on your computer. '''Note:''' You may have to enable '''Show Hidden Folders / Files.''' Enter this in the address bar; Windows: %AppData%\Mozilla\ C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\ http://kb.mozillazine.org/Profile_folder_-_Firefox#Windows Linux: ~/.mozilla/ http://kb.mozillazine.org/Profile_folder_-_Firefox#Linux Mac: ~/Library/Application Support/ http://kb.mozillazine.org/Profile_folder_-_Firefox#Mac Rename the '''profiles.ini''' file by adding '''.old''' to it. Now start the Firefox.Profile Manager. '''https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles''' '''Windows: 32-bit''' C:\Program Files\Mozilla Firefox\firefox.exe -p '''64-bit''' C:\Program Files (x86)\Mozilla Firefox\firefox.exe -p '''Mac:''' Navigate to '''/Applications/Utilities.''' Open the Terminal application. In the Terminal application, enter the following: /Applications/Firefox.app/Contents/MacOS/firefox-bin -p '''Linux:''' In '''Terminal''' run: firefox –P For users who have one or more builds from mozilla.org this will not work. It would be best to have the full path so if you had placed the untared contents in a folder in /home it would be /home/<username>/foldername/firefox/firefox –P An easy way would be not to do it in a terminal but in say a launcher since you have to point it to the firefox script in Firefox folder you can then add –P on end. You can also have the exact "profilename" after the -P so Firefox will start with that Profile. ----------------------- Select your old profile. If this works, please open the '''profiles.ini.old''' in a text editor. Copy its contence, and post it in the reply box below.