A way out of a trap page is to (force it if needed) Close Firefox. Wait about 20 seconds for Firefox to finish closing. Right-click on the Firefox icon and select Open New Window. +++++++++++++++++++++++++++++++++ Sometimes this works. If it opens, close it by Menu > File > Exit. This will flush out that web page from the Session Store.

Go to the Mozilla Add-ons Web Page {web Link} (There’s a lot of good stuff here) and search for a good ad blocker.

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

I actually browsed on another browser until I found a way to change the registry key from true to false in "Restore previous session."

Simply clicking "restore precious session" to 'no' in the content bar did NOT stop previous session and frozen window from opening again after Win10 restart.

Had to kill the 'restore previous session' in the registry. (I was able to find that info and instruction page by Googling. Sorry but I don't have it any more.

It might have just simply been a malvertising Ad on a page you were viewing.

That's exactly what it seemed to be. Thanks !