Bartlomiej, Sam was asking for you to check the logs in your _Firefox profile_ directory. There should be a "sessionstore-logs" directory. You need to get the exact path by typing "about:profiles" in the urlbar, and then you can see the path for the profile you've been experiencing this problem with. There is nothing personal or identifying in the session restore logs btw. Can you confirm you currently have "Open previous windows and tabs" selected in about:preferences? How many tabs are you typically working with? One point to make about how you are quitting the browser. Closing the 'x' button technically is just closing the window, even if its the only (or last window). It should technically show pinned tabs in there, unless you happened to close a window with no pinned tabs most recently, see bug 1717867. If the next time you close using the x button and lose your tabs, can you check if you can still retrieve them by going to History -> Recently Closed Windows -> to reopen any windows from your previous session?
Bug 1978084 Comment 8 Edit History
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Bartlomiej, Sam was asking for you to check the logs in your _Firefox profile_ directory. There should be a "sessionstore-logs" directory. You need to get the exact path by typing "about:profiles" in the urlbar, and then you can see the path for the profile you've been experiencing this problem with. There is nothing personal or identifying in the session restore logs btw. Can you confirm you currently have "Open previous windows and tabs" selected in about:preferences? How many tabs are you typically working with? One point to make about how you are quitting the browser. Closing the 'x' button technically is just closing the window, even if its the only (or last window). It should technically show pinned tabs in there, unless you happened to close a window with no pinned tabs most recently, see bug 587400. If the next time you close using the x button and lose your tabs, can you check if you can still retrieve them by going to History -> Recently Closed Windows -> to reopen any windows from your previous session?