Are you clearing your recent history when you close Firefox?

Firefox stores App Tabs and tab group details in the same file that stores details of browsing sessions. Make sure that you have not set Firefox to delete browsing history. For details of the setting to check see https://support.mozilla.com/kb/Clear+Recent+History

You will also lose them if you manually clear browsing history.

Nope, history is not set to clear, and I don't clear it manually.

If you create app tabs now, are they lost when you restart Firefox?

If they are there may be a problem with the file that stores details of app tabs and panorama groups.

To fix that open your profile folder, for details of how to find it see https://support.mozilla.com/kb/Profiles

Now close Firefox, then rename sessionstore.js to sessionstore.old. Restart Firefox and create some app tabs and see if they are still there when you restart Firefox.

They don't, and panorama was saved this time as well. I can chalk it up to a one-off occurrence. Thank you!