https://support.mozilla.org/en-US/questions/1043809

That's frustrating.

When you get your home page, does the History menu offer the option to Restore Previous Session, or is it grayed out?

If it's grayed out, are any previous windows or tabs listed on the menu that you could re-open?

Have you noticed any pattern as to when this happens, for example, after shutting down and restarting Windows? I ask about that because some cleaning utilities run at Windows startup and might target history files including your last session.

The "restore Previous Session" option is grayed out. However, it does still show me my history.

This happens to me, and I haven't found a pattern yet. I have Norton 360 installed and ratedr_jea's link brought me to another link and N 360 seems to be a possible culprit, so I'll investigate that.

This can happen if there has been a problem with creating the sessionstore.js file the last time you closed Firefox.

Use "3-bar" menu button > Exit (Power button) (Windows: File > Exit; Mac: Firefox > Quit Firefox; Linux: File > Quit) to close Firefox if you are currently doing that by clicking the close X on the Firefox title bar.

The sessionstore.js file is created from recovery.js when you close Firefox and is removed and copied to previous.js when you start Firefox to be able to restore the previous session.

You can backup the session files in the sessionstore-backups folder in the Firefox Profile Folder to make sure not to lose possible important session data.

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).

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

I replaced my sessionstore.js with previous.js and it seems to remember my tabs for now, I'll report back to see if it keeps working in the longer term (1, 2 weeks maybe)

Well, it happened again (shows only a new tab on start up), but I also noted that there sessionstore.js was missing. Any ideas?