I seem to remember that DOM Session Storage and Session Cookies are pretty big offenders. Other websites (e.g. Google Docs) use lots and lots and lots of hidden forms to remember lots and lots and lots of data.

Also, people who keep Session Restore tabs open (you know, the tab that lets you restore your session, when you have crashed) and continue browsing – Firefox needs to store several nested Session Restore JSON files.

I've always been impressed by that, because it's essentially bullet-proof.

I wish it were, but every now and then, I restart Firefox and get empty, title-only tabs that go to about:blank when I reload them, completely losing the tab. I have to use a Python script to dump the session data (which still appears in the JSON file, even though Firefox won't use it), eyeball the list to figure out which tab was broken, and reopen that URL manually.