|1. Open Firefox normally.|
2. Go to View >> Toolbars >> Customize
3. To restore your forward and back button then click the “Restore default set” button on the newly opened window.
But the above solution could be temporary. If you face this problem again in future, then go for the Method 2.
Note: The localstore.rdf file your firefox profile is corrupted. You need to remove the corrupt localstore.rdf file so that Firefox can generate a new copy. Any toolbar customizations will be gone when you next restart Firefox but future changes will be retained. Before continuing, you may wish to save a screen capture (print screen) which may help in restoring your toolbar customizations.