Window sizes and positions are stored in [http://kb.mozillazine.org/localstore.rdf localstore.rdf] in the [http://kb.mozillazine.org/Profile_folder_-_Firefox Profile Folder]. See http://kb.mozillazine.org/Corrupt_localstore.rdf (caution: do not delete the localstore.rdf file in the Firefox program installation folder) Note: Deleting the file [http://kb.mozillazine.org/localstore.rdf localstore.rdf] will reset the customizations of the toolbars to the defaults. You can rename ''localstore.rdf'' to '''localstore.rdf.sav''' to test if that solves it. Then you can restore the customization by copying ''localstore.rdf.sav'' to ''localstore.rdf'' if it didn't work.