Note the Nightly builds are development versions for testers and can get broken at any time. Try to start Firefox in <u>[[Safe Mode|Safe Mode]]</u> to see if that works. *Windows: hold down the Shift key while starting Firefox *Mac: hold down the Options key while starting Firefox Create a new profile to test if your current profile is causing the problem. See "Creating a profile": *https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles *http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems. *http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox See also the daily threads on the MozillaZine Builds forum to discuss issues with latest Nightly builds. You need to register at the MozillaZine forum site in order to post at that forum and your first post needs to be approved as an anti-spam measure. See http://forums.mozillazine.org/viewforum.php?f=23