You think Firefox restores your tabs when you close and open it (with Tab Mix Plus or the session saver). Don't bet on it, try the following sequence:
- Open the Download window
- Open a few tabs
- Close the main firefox windows
- Open a new firefox instance (only the homepage is opened)
You have now lost all your tabs. If those contained any resident information or things you wanted to read/follow up they've been lost :-/
So if you didn't know the Download page was still open somewhere, starting a new firefox will loose your collection of tabs.