I've had 5k+ tabs open at some points, because I just don't close any of them, and I often middle click as I want to navigate back to the page I was at. Additionally, a lot of sites break the back button, like collapsing comments re-expanding, or it loads slowly and I wanted to look at it quick. Organization is pretty nice with Tree-Style Tab for Firefox.

Every few months I purge all of my tabs, but for the most part, I just don't care when I have 32 GB of RAM.