As a Windows person, I am not familiar with how virtual/multiple/extended desktops work on MacOS. Is there any way to discover whether those windows are on a different/extended desktop?

Thank you for trying to help. No there are no other desktops or mirrored screens, its just a firefox glitch.

I did come up with fix though. I forced relauched Finder and force quit Firefox. Then opened a new finder and navigated to Firefox within applications. When I opened through Applications (not through the Firefox icon in Dock at the bottom), a new page opened up allowing me to restore each previous session window individually (if I clicked restore session it once again did not work and opened up the 'phantom window').

B

So far so good. Fingers crossed that Firefox stores the new position.

On Windows, there is a file named xulstore.json that stores the coordinates of the main window (the one that opens first when you start Firefox). I've encountered a problem where the left or top position was a large negative number so the window wasn't on screen. I had to manually edit the file (or I could have deleted it) to fix the problem. I don't know whether Firefox stores window positions the same way on Mac. If so, you could look for that in the future. The file is stored in the currently active profile folder: Profiles - Where Firefox stores your bookmarks, passwords and other user data.

I'll keep it in mind, thank you!