The problem is that you're doing something that uses lots of tabs, and with luck you remember to close them down. Still, at the end of the day, you may have an extra five tabs open, and that's 100 a month. After a few months you have 500 tabs.

So then you devote a couple of hours to going through and closing tabs, and find you still have 300...

The solution, if anyone ever gets this far down the page, is to use Session Manager to save the tabs, close Firefox, and then just uncheck the boxes for the tabs you don't ever want to reload.