Hey, its not a silly question, the problem you are talking about is not a problem with Firefox, I think it is a Windows feature. I can't remember where the option is, but somewhere, you can select to have any window you previously had open revert to that state once its open again. I used to have the same problem with windows folders that would not open full screen. It took me awhile to figure out how and why, but I believe this solution might work...
Firstly, how do you exit Firefox? Is it with the red X in the right hand corner? If it is, that maybe the problem. The solution is simple, make sure you window is maximised, and the go to file>close (or close window). Now try opening it again, and it should,

open as you left it. I hope that helps. 👍