If you close the sidebar and close Firefox then the sidebar should stay closed. Note that this is a global setting that applies to all Firefox windows, so best is to leave only one window open when you close Firefox. If this doesn't work then rename/remove xulstore.json (xulstoreOLD.json) and when present delete localstore.rdf in the Firefox profile folder with Firefox closed. You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the <b>about:profiles</b> page. *Help -> Troubleshooting Information -> Profile Directory:<br>Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder *http://kb.mozillazine.org/Profile_folder_-_Firefox