Currently, if you close a window with tabs (Main session) but have another window with tabs (session #2), all "restore" options do not contain the Main session. Further, if you close that session #2 also and re-open Firefox, you will only be given the option to restore session #2.
More often than I'd like to admit, I will lose my master session to the ether because I opened a separate window and forgot to close it before closing the main session window. I will immediately attempt to re-open it, but "recently closed windows" contains none from the main session, and neither does "recently closed tabs".
This is the one way Firefox has been tripping me up for years.
Firefox only seems able to restore your lost session if all windows have been closed and the last window/session which was closed is the one you want to restore. If you've still got another window open (perhaps because you dragged some windows out to compare data side-by-side) then your main session is gone forever, never to be seen again except for the last few dozen individual tabs you can find in Recent History.
Stopping this from happening would be such a Quality of Life improvement for me. It has stopped me in my tracked on and off for years.
This issue may even be solved if a solution was implemented for my request for persistant sessions to become a thing: https://connect.mozilla.org/t5/ideas/persistent-sessions-open-all-some-open-tabs-on-another-device/i...
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There is a feature that tracks closed windows on the 'History' drop-down, as shown below with 'Recently Closed Windows' selected.