It is possible that there is a problem with the files sessionstore.js and sessionstore.bak in the Firefox Profile Folder.

Delete the sessionstore.js [2] file and possible sessionstore-##.js [3] files with a number and sessionstore.bak in the Firefox Profile Folder.

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

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

See also:

• [1] http://kb.mozillazine.org/Session_Restore
• [2] http://kb.mozillazine.org/sessionstore.js
• [3] http://kb.mozillazine.org/Multiple_profile_files_created

I did not find a .bak file, but found four sessionstore-x.js files, where x was 1 thru 4. I deleted all those files (after closing ff) and then I opened up ff, opened up three windows, did a file,exit, and relaunched ff. only the home window came back, and the History, Restore previous session was still grayed out.

Make sure that you do not use "Clear Recent History" to clear the 'Browsing History' when you close Firefox.

• https://support.mozilla.org/kb/Clear+Recent+History

I most generally just crash firefox via task manager because otherwise if I just file,exit, or click on the X I don't get my open windows and tabs back. But, no, I don't clear history when exiting ff.

A whole bunch of people found a solution to this by editing the Firefox config file. See http://www.virtualhelp.me/applications/51-office/288-firefox-4-doesnt-save-tabs, (for versions 4 thru 10) or, for a start:

"A. By default this function has been disabled. To enable it, do the following: 1. In the address line, type in "about:config" (without quotation marks). Click on "I'll be careful, I promise!" 2. In the filter field, type in browser.showQuitWarning. This should filter out all other options 3. Double click on the option that is left in the list. The value should switch from "false" to "true". 4. Close about:config tab and it should work PS: I have no idea why they did that."

  As I said, that did the trick for many, tho some apparently had to change a couple other settings as well.  See the post...