I upgraded from 126.0.1 to 127 today. Upon update, all my windows are gone. The only way I could access them is to revert to an older profile backup and reinstall 126.0.1. I tried exporting my session with Tab Session Manager in 126.0.1 and reopen them in 127, but that only restores the last tab of each window (oddly). The json clearly lists 3 windows each with multiple tabs. I tried copying the sessionstore-backups folder, but no dice. Symptoms similar to : https://support.mozilla.org/en-US/questions/1446921