Fix/Report SessionStore leakage
In #4430 (closed), Some Guy is quite insistent that the session store still leaks in Windows, and has a repro case.
S/he claims that all you have to do is close Tor Browser with a tab open, and it will write the title of that tab to sessionstore.js
I think this is a Firefox bug. I wonder if it happens in Private Browsing Mode. I guess I need to unmothball my Windows VM and see if I can repro, too.