I categorize this add-on as *essential*, at least for people who use more than a handful of tabs, and/or who don't like wasted RAM. **Ever since installing this, it's as though I installed another 2 GB of RAM. ** It should be built in to Firefox. I wrote angry letters to Firefox about their reckless waste of RAM, and even suggested they free up memory exactly as this add-on does. Firefox is nasty -- it will keep on using RAM till there's 98% memory in use and the whole machine grinds to a halt. I was on the verge of abandoning Firefox over this. No installation of FF is complete without this functionality.