I came across PermaTabs today thanks to the good folks at LifeHacker. The Firefox 2.0 extension adds the ability to set any tab into a permanent tabs ("permaTabs"). PermaTabs can't be closed accidentally (even if you're closing multiple tabs at once), and are persistent across sessions. If, like me, you accidentally close a tab you really need open all the time in a frenzy of close button clicking, you'll immediate grasp the "must-have-ness" of this tiny (11 Kb) add-on.
Just so you know, PermaTabs opens three random permatabs on installation. Don't panic - you're not stuck with them - simply right-click on the permatab and deselect the Permanent Tab option and you can close it like any other tab. My random set included Technorati, eBay, and the BBC home page if that sets your mind at ease about what kind of randomness you might expect.
Update: Many of the comments on the LifeHacker post take issue with the usefulness of this add-on, claiming quite correctly that Tab Mix Plus and other extensions provide similar functionality. While that's true, this extension uses much less memory and is far less bewildering that Tab Mix Plus for most users. YMMV, as they say but I think this will be enormously helpful for some Firefox users who prefer not to load up their browser with a ton of add-ons or deal with really complex preferences settings.