Try create a new perfil or reset Firefox:

• Create a new profile
• Reset Firefox

You can check browser.sessionstore.* prefs including browser.sessionstore.resume_from_crash on the about:config page and reset browser.sessionstore prefs that are user set (bold) via the right-click context menu to the default value to make sure that Session Restore is enabled and working properly.

• http://kb.mozillazine.org/Session_Restore
• https://support.mozilla.org/kb/restore-previous-session
• http://kb.mozillazine.org/browser.sessionstore.resume_from_crash

You can check for problems with the sessionstore.js and sessionstore.bak files in the Firefox profile folder that store session data.

Rename (or delete) the sessionstore.js file and possible sessionstore-##.js files with a number and sessionstore.bak in the Firefox profile folder.

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

Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost and you will have to recreate them (make a note or bookmark them if possible).

Thanks for replying! Diego Victor, I tried Resetting Firefox. After the reset I still had the same problems. I haven't tried creating a new profile yet, but it's next on my list.

cor-el, all of my browser.sessionstore.* prefs are set to the defaults, I'm not sure how to be certain that they are working properly past this. I followed your instructions for finding my Firefox profile folder and renamed the sessionstore.js file, then started up firefox. After all of this the problem still persists!

Interestingly, the administrator of this machine (this is a work machine) logged into the admin account and verified that "Undo Close Tab" worked for him in that account. I've now reset Firefox, and all my extensions are no longer present, yet I'm still having this issue. Are there some other defaults I can modify or something else I can check, or should I just go ahead and have firefox completely removed and reinstalled on this machine? Would that be the best bet to fix the problem if some profile information is causing this? Might this be related to history or privacy settings (which I don't believe that I have changed)?

Thanks.

It would be best to use the Profile Manager to create a new profile if there are issues with files that are preserved via a reset (cookies, open tab are among those). In such a case you can never be sure that the issue isn't carried over via a reset.

On Windows there is also a high chance that security or other software is interfering with files in the Firefox profile folder, so make sure to disable all such features in this software that might corrupt files in the profile folder.

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) as a test.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/