My favourite feature is the fact that Firefox will save the session and load it back up the next time you open it. Not just the page(s) you happened to be on at the time you shut the browser either, it'll be exactly as you closed it last, including all the pages you browsed before and after in each tab. Amazing stuff. No more bookmarking every single page you need to revisit maybe once in the near future.
Therein lies the problem.
I keep leaving tabs upon tabs open because I need to do a follow-up of some sort at a later time, and being lazy as I am, I leave the following-up until too late and I do something stupid to lose all my tabs. And of course, I'm lucky if I remember even one of the sites that were there.
Damn. I had a whole series of tabs open that I wanted to download things from. Aargh. I could go through my history to figure it out, but guess what. I keep 60 days worth of pages visited. I don't even want to think about how long it's going to take to sift through all of that. Last time I just kinda gave up and forgot about it.
Firefox is an awesome browser. Obvious win over IE - better security, visually more appealing, way more features, much more user-friendly and efficient. Microsoft totally jumped on the tabbing bandwagon for IE7, and eons too late at that.