You can check if you can find a upgrade.js-<build_id> file in the sessionstore-backups folder in the profile folder.

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

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

You will normally find these files in the sessionstore-backups folder:

previous.js (cleanBackup: copy of sessionstore.js from previous session that was loaded successfully)
recovery.js (latest version of the sessionstore written during runtime)
recovery.bak (previous version of the sessionstore written during runtime)
upgrade.js-<build_id> (backup created during an upgrade of Firefox)

You can copy a file from the sessionstore-backups folder to the main profile and rename the file to sessionstore.js to replace the current file (make a backup copy of the current sessionstore.js).

See also:

• http://www.ghacks.net/2014/06/27/mozilla-launches-improved-session-restore-firefox-33/

Hmm, the last major update to Firefox 42 was released around November 3rd. Unless you were holding back for a long time, I'm not sure this problem is linked to a Firefox update.

Did you update Windows recently? Sometimes after updating, Windows starts up with reduced permissions and Firefox cannot access all of its data. This usually is easily fixed by shutting down Windows and starting it up again, so before doing anything drastic, I would try that.

Sometimes after an update Firefox will report that it is starting slowly and you can speed it up by restoring some default settings. If you click that button, the Refresh feature runs and your Firefox settings (profile) folder is moved to the desktop inside an Old Firefox Data folder. Do you see anything like that? If so, hang onto it in case we need to dig into it to restore some missing data.

If the problem is with missing tabs for the pages you had open in your previous session, please see cor-el's reply.

If the problem is that you no longer see the buttons on the Bookmarks Toolbar, you can use one of these methods to show the toolbar list and turn that bar on there:

• right-click a blank spot on the tab bar (or the "+" button)
• tap the Alt key to activate the classic menu bar > View > Toolbars
• in Customize mode > Show/Hide Toolbars (see: Customize Firefox controls, buttons and toolbars)

To turn on the Bookmarks Toolbar, or other bars, click it on the list.