You may have corrupt sessionstore [v56] sessionstore.jsonlz4 file(s). Delete all sessionstore* files and the sessionstore-backups folder.

Type about:support<enter> in the address bar.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder

Linux: Under the page logo on the left side, you will see Application Basics. Under this find Profile Directory. To its right press the button Open Directory.

Locate the above file. Then rename or delete it. Restart Firefox.

Don't delete the files if you need to rescue any data from them, just move them out of the profile folder to some location where Firefox doesn't look for them. You can try to read out their contents using this tool: https://www.jeffersonscher.com/res/scrounger.html

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox Title bar.

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

On Mac things can be more complicated because you can close all open windows, but keep Firefox running with the menu bar visible or minimized in the Doc.

You can check "History -> Recently Closed Windows" to see if you can reopen a closed window although Firefox only keeps a record of three windows (about:config => browser.sessionstore.max_windows_undo).