Every so often when I open firefox, two windows will open. One window has all my saved tabs, the other is glitched and is semi-transparent. The glitched window won't load or do anything, but closes out fine. However, then when I try to close the other window with my tabs, I get told the "You are about to close x tabs" message. No matter what I try, it won't let me save them. To make things worse, even if I close it out and open a fresh window, it won't save those tabs either. This goes on till I uninstall and reinstall, then firefox saves tabs again.... until it glitches again. This problem has been going on for me since this past summer, and I'm getting sick of losing everything every couple weeks. And from what I've seen I'm the only one having this problem? I tried all the solutions from similar problems, none of them worked.
In Firefox 23, as part of an effort to simplify the Firefox options set and facilitate future improvements to Firefox, the option to hide the tab bar was removed.
Fortunately, this can easily be resolved if you desire the keep tabs hidden. You can install "Hide tab bar with one tab", an extension hosted on Mozilla's add-ons site, which will restore the ability to hide the tab bar.
When you are closing the window, are you using the "red X" button? I think in that case, it's normal to get the error about closing multiple tabs. To ensure that your session ends with that window and all its tabs still "open," it's better to exit Firefox using the "Exit" item on the menu:
orange Firefox button > Exit
classic File menu > Exit
But as for why the other window appears and what's wrong with it, I'd be guessing and with something that intermittent, I don't have any good guesses at the moment.