"There are some invalid URL" when trying to save a session. That's not true, all are valid.

"Window Session Saver" by Aayush Gogia, Rupesh Patil is HORRIBLE; an utterly worthless extension for Firefox "quantum". : : : : This pathetic extension does NOT save sessions properly at all. It will randomly save only a very few of the tabs. You have several dozen tabs open it will only open one or two of them. It will not even open sessions where is claims that some of the URLs are no longer valid. Why the hell does that matter? If one or more of the URLs are bad that's my personal problem, I don't need this idiotic extension refusing to open all the other tabs just because of that. There's no way to edit the name or URLs of previously saved sessions. Uninstalled. AVOID this add-on at all costs!

How could I rename or delete/remove an individual session? Where are they saved?