Sessions are considered part of history. Could you check whether Firefox is set to delete history at shutdown? You can check that here:

• Windows: "3-bar" menu button (or Tools menu) > Options
• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

In the left column, click Privacy & Security

On the right side, scroll down to the History section. With the setting "Firefox will: Use custom settings for history" do you see a checkmark on this preference:

[X] Clear history when Firefox closes

If so, you either could turn that off, or click the Settings button to the right of it and make sure you are not clearing Browsing & Download History.

There also are third party programs that may "clean" browser data between session, including potential CCleaner and Advanced SystemCare. Both should have a way to turn off cleaning Firefox files.

Also, as a possible temporary workaround, I have a tool to extract the URLs from your old session history files:

https://www.jeffersonscher.com/ffu/scrounger.html

After loading your file into the page, click the Scrounge URLs button to read out the tab addresses. If that doesn't finish within 15 seconds, the tab probably crashed. You could close it and try again.

Assuming you get a useful list, use the Save List button to archive the URLs to a web page of clickable links.

The custom settings for history was checked and I've had firefox set to delete browsing history and cache on exit as far back as I can remember. No idea why it started deleting the sessionstore files and backup folder on exit, but unchecking the custom settings seems to have fixed that problem. How it got checked to begin with is beyond me. As for the tool you mention it is a life saver. I just had to rename the file to .js because it was saved in an uncompressed form.

Sadly I had to go back to a session from april 2020 but at least it's not deleting the sessionstore files any more. Thank you for the help!

If you use "Clear history when Firefox closes" to clear the browsing history then this prevents Session Restore from restoring tabs and pinned tabs. This has always been the case AFAIK.

Note that you only (always) see "Use custom settings for history" stay selected when you make changes to the history settings (i.e. some are not default) and you need to select this choice to be able to inspect the settings and make changes. Selecting "Remember history" or "Never remember history" (Private Browsing mode) merely resets all history settings to the default.