If you close Firefox with the sidebar closed then the sidebar should still be closed on the next start. If you have multiple windows open then try to close all but the last one to see if that works.

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox title bar.

• "3-bar" menu button > Exit (Power button)
• Windows: File > Exit
• Mac: Firefox > Quit Firefox
• Linux: File > Quit

Try to rename/remove the xulstore.json file in the Firefox profile folder.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Thanks for the fast response :-) Your answer was spot on when I am using the normal, latest version of Firefox. I have found that my problem occurs when I am using the new Firefox Test Pilot program. If I uninstall the add ons for this then the browser works as it should.

Thanks again for the help :-)

Only this worked for me: "rename/remove the xulstore.json "