First check for closed windows on the History menu, and within each closed window, closed tabs.

Second, I believe Firefox will look for the file to restore in this order:

sessionstore.js recovery.js recovery.bak ? previous.js

So you may need to hide all the files you don't want Firefox to use.

Third, you can use a script to view the contents of a session history file as a list of addresses. This is one way to confirm that the file you're trying to swap in actually has what you're looking for. It's also a way to load those pages. This thread has some tools for that: https://support.mozilla.org/questions/1164558

After copying previous.js to sessionstore.js when Firefox is closed (this is important) and making sure there are no copies of that file in your profile such as sessionstore-1.js, tabs and windows should restore instantly, but only if you click Restore Previous Session at the bottom, or when choosing that item under the History menu (or button) item. That is when Show your home page or Show a blank page is set in Options.

The Restore Previous Session item should only be greyed out when Show your windows and tabs from last time is set, not for the other options. Is there any chance of an add-on interfering, possible one with session store functionality? And does the same happen in Firefox’s Safe Mode?

If the latter does not apply, the sessionstore file could be corrupt, but I might be wrong. Since you wrote it happens a few times per year, I can tell you this also occurs on my system (often after using several windows and 100+ tabs), which is reflected by slowness, high memory use and Could not write session state file errors in the Browser Console. However that corruption did not lead to your issue - it restores but with old entries - but your file may be corrupt in some other way.

If session restore works for you as it should after opening a new session, closing and restarting Firefox (which will replace the file with an empty default file) or by deleting all copies and the current js file, I think you can probably assume there is no other issue in the profile and the file is corrupted allright as well as its copies, though it would be rare. You can at least try this to make sure this is the case. If you need vital info such as visited pages, you could try opening it with an editor to find the links.

If on the other hand you proved there is still an issue with session restore even with new session and hence sessionstore file, there may be other reasons for this issue and a solution that you can find in this question.

Also see: Profiles - Where Firefox stores your bookmarks, passwords and other user data Restore previous session - Configure when Firefox shows your most recent tabs and windows Firefox hangs or is not responding - How to fix

Does this help?

The Restore Previous Session item should only be greyed out when Show your windows and tabs from last time is set, not for the other options. Is there any chance of an add-on interfering, possible one with session store functionality? And does the same happen in Firefox’s Safe Mode?

Sorry I don't get on much durring the week

YES it is set for show tabs from last setting!

So how do I get my tabs to reappear if not set to that?? since all it offers is firefox home/ blank page/ and show tabs!

and how do you get firefox safe mode?

and I have a "NEW" firefox opened with 4 tabs and the only sessionstore.js in the profiles is the one from the 16th so where is this one hiding or is that working off the previous tab session setting??

While I can understand some of the answers here and figure out how to get them to work I fully admit I'm not the smartest one on the planet.

Given that you new how to copy sessionstore files, I assumed you were familiar with Firefox’s Safe Mode. :)

You can start it by holding Shift (for Windows) or choosing Help > Restart with add-ons disabled - it will disable any add-ons that may be causing the behavior. It is not uncommon for session related extensions to cause this type of issues - disabling add-ons one bye one in regular mode is the way to find out. You can also disable all of them in regular Firefox mode instead and enable them one by one, since you will probably need to restart Firefox a few times to find out what the Session restore does so you would need to do so in Safe Mode every time.

If you (temporary) set the Home page setting to Show your home page, you should only see one tab with that page after starting Firefox, not 4, and no, it should not be working off the previous tab session setting. If the sessionstore file is not updated after closing Firefox, I’d say that would indicate an issue with your session storage, possibly by corrupted sessionstore files, or an add-on.

If Firefox still opens with more tabs, I would first remove all files within the sessionstore-backups subfolder of your proflie folder and the sessionstore.js file itself. If that does not bring back default session restore behavior, have a look at this KB article. Also take note of the last article mentioned above and look for "duplicate" in order to find if you might have duplicate sessionstore files in the profile.

You could also test if a new profile offers normal behavior. If it does, you can be sure the issue is within the profile.

Please let us know how things go.