If the browser crashed or did not close properlly, it will restore the last session. This is how the browser is set up.

Make sure you close the browser using; Windows: Menu > File > Exit Mac: Menu > Firefox > Quit Firefox Linux: Menu > File > Quit

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

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

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

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.

Thanks for the responses.

As for the close issue this raises the question of why I've always been able to close using the upper right hand corner x without having this problem before.

I followed your directions in your other response and found only a session store back up file, which I deleted. This seems to have solve the problem. Thanks!

Glad to help. Safe Surfing.