File > Exit or Firefox button > Exit

Since you apparently want to come right back to where you were, you could upon restarting Firefox restore previous session via the History menu.

Under the same circumstance I might just terminate Firefox with the Windows Task Manager and let Firefox restart automatically with my previous session, since that is how mine is set up. Either way you should not lose anything you were working on including filling in of forms. I'm sure the second method would not be officially/unofficially approved by Mozilla or by Ed but it works for me, and I might not remember that I had anything being worked on otherwise.