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

Might be this:

There is an important difference between Windows and Mac in what happens when you close windows.

On Windows, when you close the last Firefox window, Firefox exits automatically. Your last window is considered to be "open" for session restore purposes.

On Mac, when you close the last Firefox window, Firefox continues to run without any windows. Your windows might be lost, or you might need to use History > Recently Closed Windows to get that window back.

The safe way to Quit Firefox on Mac and save your last window for your next session is to keep that last window open and quit using the menu. Either:

• "3-bar" menu button > Quit
• (menu bar) Firefox > Quit