You can check for problems caused by a corrupted xulstore.json file and delete the xulstore.json file in the Firefox profile folder.

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

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

Try to reset the browser.uiCustomization.state pref on the about:config page via the right-click context menu.

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

You can also check for problems with preferences.

Delete possible user.js and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in the current Firefox release.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences

Thank you cor-el! Unfortunately none of this solved my problem. I did some tests: Actually Firefox resets itself to a fixed state no matter what changes I made to the toolbar appearance, even if i restore it to its default. And it does this upon quitting, i.e. I see the toolbar appearance changing the moment I quit the browser.