If you quit from the menu or press Ctrl+Q then all windows should be restored next startup. However if you close them individually then a maximum limit of three may apply. You can increase this by going to about:config and changing the value of browser.sessionstore.max_windows_undo.

Thank you, that was helpful. Unfortunally someone at Mozilla/Firefox has changed the value to 3, what the number was before I updated I dont know but I have had 11-12 windows open for a couple of years now and havn't had any problem before. Perhaps someone could check that so I will be, if not alone with my misfortunate update, one of few that is affected. There was no reason to change the previous value, none what so ever. I guess that getting the windows back is not to think about, right? I always turn of my computer and let Firefox close by itself. I changed the value to 12 and restarted and my by now 4 windows turned up. And please tell that person to not change anything again.

I looked in History, closed windows and it is grey, empty. Should the closed windows not appear there, I mean they are closed, right? And this is both before with the 3 max and after change to 12 and restart.

The window undo limit has been three for a long time. Increasing the limit won't bring back windows retroactively. There is no limit for windows that are considered to be actively open which is why it is best to quit with Ctrl+Q.

OK, I think that I must have had a really old version then. But how do I get my 9 windows from 202309 back? I found a file upgrade.jsonlz4.20230928054334. How can I use it?

I copied the upgrade.jsonlz4.20230928054334 to sessionstore.jsonlz4. I thought that would have taken care of my problem but the file only had three windows when I started Ff. So I guess that the upgrade file was updated using the new value in browser.sessionstore.max_windows_undo. I guess that is a bug, not making a backup of reality (7 windows) but using a new value that says that there can be only 3. So I have this useless upgrade backup from 2023 with 3 windows and I have an older backup (triwd it too) from jan 2022 with 9 windows. I suggest that someone takes this to bug section.

If there is someone that can help me get my 7 windows back i would really appricate it. Perhaps there is another file I can use?

Hopefully someone here is more experienced than the answers so far shows. If this is the best support one can get from Mozilla then I have to look up migrating to another webbrowser. I really liked Firefox but every time I have updated something has gone wrong. If the same people that programs Ff are the same that give support then I say I understand why there is problems. If you know that you can't contribute to the solution why not consider to not write meaninglessnesses. Hope I havn't stepped on anyones toes.

the file only had three windows when I started Ff. So I guess that the upgrade file was updated using the new value in browser.sessionstore.max_windows_undo.

There is no limit on open windows in the session. If it starts with three windows then there could still be up to three additional windows in the "Recently closed windows" list (Ctrl+Shift+T).

"There is no limit on open windows in the session. If it starts with three windows then there could still be up to three additional windows in the "Recently closed windows" list (Ctrl+Shift+T)."

We are not living in the same reality I guess. The only thing that PERHAPS takes care of this is make a backup at shutdown of computer by calling a script.

I have found a resonable backup that was taken just 3 days before update of Ff that started all this. Thank you for nothing.