From MozillaZine Knowledge Base
If you have a backup copy of the localstore.rdf file from a working profile, a quick remedy is to close your Mozilla application and overwrite the localstore.rdf file in the profile folder with your backup copy (see Profile folder - Firefox or Profile folder - SeaMonkey for profile folder locations).  If you have no localstore.rdf backup, you will need to close Firefox (or SeaMonkey) and remove the localstore.rdf file from the Firefox (or SeaMonkey) profile folder (by moving, deleting, or renaming the file). When you restart the application, a new localstore.rdf file will be generated. Your toolbar customizations will be gone but future changes will be retained.
Before continuing you may wish to save a screenshot of your current toolbar customizations, to help you restore them later.
Note: You can restore the default set of toolbar items by opening the Customize Toolbar dialog and selecting "Restore Default Set" but this does not restore the default content of the localstore.rdf file and is not a solution if the localstore.rdf file is indeed corrupt.
Use either of the two methods given below, to regenerate the localstore.rdf file.
Safe Mode method
Note: In Firefox 15 and above, Safe Mode has been redesigned to replace the individual troubleshooting options that were included in earlier Firefox versions with the option to Reset Firefox. Resetting Firefox will automatically create a new profile which will include a new copy of the localstore.rdf file; however, using the Reset Firefox feature will also remove all of your other Firefox profile data and settings, except for certain essential data (bookmarks, browsing history, passwords, cookies, and form-fill data) that is migrated to the new profile. The old Firefox profile will be deleted and all of the old Firefox data will be moved to a folder on the desktop. Warning! This change cannot be reversed without manually transferring the old Firefox data back to the new profile.
Firefox 15 and above:
Firefox 14 and below and SeaMonkey:
Related bug reports: