Do you have a problem with tab hoarding? I sure do. Firefox is faster and more usable when the number of tabs are few, so Firefox should help us clean up the tabs we're not using.
I wrote this add-on to test a theory: If you have an open tab that you haven't used in two days, that page is probably important enough that you want it bookmarked, but not important enough to be a tab. The Grim Tab Reaper will turn it into a bookmark and close it for you. (48 hours is the default, but the exact period of time is configurable.)
The Grim Tab Reaper never closes app tabs, or the active tab. Tabs that are "reaped" get tagged with "tabreaper", so you can find them easily by looking under "recent tags" in the bookmark menu.
Try it out and let me know what you think about the experience of tabs having a limited timespan.