This situation occurred unexpectedly a few days ago. I had auto-updating turned off since I have a slow connection. However, in going through the preferences AFTER the corruption started I noticed that auto-update was now turned ON. I was not playing with this; I had no reason to do so. I have used a tab manager extension, Tab Mix Plus, for a long time without problems. The progression went as follows: The tab marker content, organization and operation was different than my specified preferences. One or more of the menu, nav and bookmark bars would sporadically disappear or become inoperable or inaccessible. They could sometimes be resurrected by hiding and unhiding, but in the nav bar especially, the buttons remained inaccessible. Sometimes at startup a menu bar would appear on the left side of the window. I have NEVER used this or even know how to set it up. The main context menu content was changed and some options became inaccessible or did nothing when clicked or when clicked, produced no action. Tab and window management did not function as expected. ------------------ Failed Remedies Closing and restarting Firefox changed nothing. When opening the Edit > Preferences > Tabs > Tab Mix Plus Options panel, the first subpanel that appeared was always "Session" which, in part, contained the following message: "Tab Mix Plus detect [sic] Session Manager Extension is installed. Tab Mix Plus session manager will be disabled." I didn't fool with anything here because I had not installed this Extension and it wasn't in the list. I didn't know how to disable it or what effect it was having in the first place. I couldn't reset any of the preferences in the Tab Mix Plus panel because none of the buttons at the bottom of the panel (active or not) would operate except Cancel.