Check these tips. https://support.mozilla.org/en-US/kb/how-do-i-restore-my-tabs-last-time https://support.mozilla.org/en-US/kb/how-restore-browsing-session-backup https://support.mozilla.org/en-US/kb/restore-previous-session https://support.mozilla.org/en-US/questions/1204253 https://support.mozilla.org/bm/questions/1267353

Did you already reopen Firefox ? How many windows did you have open ? If that is the case then check "History -> Restore Previous Session" and "History -> Recently Closed Tabs/Windows". ---- Backup the session files in the sessionstore-backups folder in the Firefox profile folder to make sure not to lose possible important session data. Do NOT close Firefox when Firefox is already running. You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the <b>about:profiles</b> page (Root directory). *Help -> More Troubleshooting Information -> Profile Folder/Directory:<br>Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data You will normally find these files in the sessionstore-backups folder: *previous.jsonlz4 (cleanBackup: copy of sessionstore.jsonlz4 from previous session that was loaded successfully) *recovery.jsonlz4 (latest version of sessionstore.jsonlz4 written during runtime) *recovery.baklz4 (previous version of sessionstore.jsonlz4 written during runtime) *upgrade.jsonlz4-<build_id> (backup created during an upgrade of Firefox) You can copy a file from the sessionstore-backups folder to the main profile and rename the file to sessionstore.jsonlz4 to replace the current file with Firefox closed. *make sure to backup the current sessionstore.jsonlz4 You can look at this tool to inspect a compressed jsonlz4 sessionstore file. *https://www.jeffersonscher.com/res/scrounger.html