I'm honestly now sure how best to explain this because it's kind of weird, but I will try my best. Basically, Firefox thinks there's two windows open, when really there is only one. When I right click on the Firefox logo in my Dock (I'm on Mac), it tells me I have two windows open, the main window I use and then a second "What's new with Firefox" window (see screenshot). This is the "invisible" window. I have tried many things to get Firefox to realize that window doesn't exist. I've quit Firefox many times, tried to get my Mac to display all windows (it says there are no open windows when I close out of my main window), restarted my Mac, cleared cache and cookies, removed from Dock, etc. At this point, I don't know what's left to do other than remove Firefox entirely and re-install it and I would hate to do that. I hope I've provided enough detail to get the gist of the issue. If you need any more info, I'm happy to provide it! Thanks in advance for your help.