Do NOT take any option that says something like remove personal settings, because that is likely to remove profiles from all normally installed versions of firefox resulting in the loss of bookmarks, passwords, and other settings. It is too late to help now, but it is always good practice to somehow back up your bookmarks e.g. manually * see [[backing up and restoring bookmarks]] Firefox does try to keep backups, but they are in your profile, and can get corrupted or deleted in some circumstances * see also [[lost bookmarks]] Can you post the exact wording of the message it may help in working out what is happening ? It will certainly be worth starting in Firefox's [[safe mode]] by for example holding the shift key as you start, or using the menu option. * do not make any changes on the first option window you see, just click on continue * use only the default theme * if it does not improve then whilst still in safe mode temporarily disable all plugins See also * http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox <br> note in particular the mention of the add-on to rebuild the database * [[Firefox is already running but is not responding]] It may well be a sensible precaution to preserve the current situation/data by backing up your profile by simply copying it and its folders and files to some location such as your desktop. (Quickest way to find its location firefox button -> help -> troubleshooting information -> profile [open containing folder] )