My wife has exactly the same issue with Firefox and she leaves loads of tabs open for silly lengths of time as well. After a while Firefox has used up all available memory or plugin-container is acting screwy. I think I've finally convinced her to scale back her long term tab usage, that and she uses Chrome to watch videos.
Some people have said "put I have 100 tabs open while I'm doing xyz!", yes, that's fine, just don't leave them open for days on end. Firefox probably isn't the best at handling long-term browser sessions, but IMO it is just lazy and is asking for trouble to leave stuff open for that long, either through resulting browser stability issues or at some point it crashes and for some reason the sessionstate file is corrupted. Sometimes I might have 50 or more tabs open but once I'm done I close them and bookmark any that I have to refer back to.
The bookmarks facility is there for a reason. With a bit of organisation it is very easy to use. In my bookmarks list I have an area where temporary bookmarks are kept (then deleted once they're no longer needed) and the rest are filed into separate folders.
Browsers have to handle an awful lot of crap, probably a lot more than any other bit of software. In a word processor you might open a document made in another package, perhaps with a bug or two in, but that's a single document and people don't tend to leave them open for days on end (the 'save early, save often' tactic is used by most sensible people), whereas website designers do all kinds of crazy things, as well as people having say ad banners which try to exploit possible vulnerabilities in the browser. People who have a multitude of tabs open are asking their browser to handle situations like that are just multiplying the likelihood of a problem.