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]. Delete localstore.rdf or rename the file to localstore.rdf.sav in the [http://kb.mozillazine.org/Profile_folder_-_Firefox Profile Folder] to test if the file is corrupted.<br /> See http://kb.mozillazine.org/Corrupt_localstore.rdf (caution: do not delete the localstore.rdf file in the Firefox program installation folder) Note:<br /> Deleting the file [http://kb.mozillazine.org/localstore.rdf localstore.rdf] will reset the customizations of the toolbars to the defaults.<br /> You can rename "localstore.rdf" to "localstore.rdf.sav" to test if that solves it.<br /> Then you can restore the customization by copying "localstore.rdf.sav" to "localstore.rdf" if it didn't work.<br /> ---- You can also try to move (delete) the Firefox plist file. See [http://www.macfixit.com/article.php?story=20060606082246983 How .plist files become corrupt and troubleshooting the results] Go to "Library > Preferences" and remove the plist file for Firefox (org.mozilla.firefox.plist).