Tab Mix Plus is a Firefox extension that adds features like tab duplication, merging, focus control, and extended undo options to Firefox. You can also set it to force Web sites that want to open a new window to instead open a new tab. I find that last especially handy--we use publishing software called TeamSite from Interwoven to publish InformationWeek.com, and it frequently wants to open a new window; without Tab Mix Plus, I often end up with four, five, or more TeamSite windows cluttering up my desk. I've been giving Tab Mix Plus a whirl for a couple of minutes now, and it seems to do a pretty good job keeping everything in one window. (Via Lifehacker)