Last Updated May 24, 2010 11:23 AM EDT
What's the big deal about that? Like most Firefox users, I routinely keep multiple tabs open at any given time. The problem is, I frequently need to jump back and forth between the same two tabs, a hassle when they're not adjacent.
Sure, I could drag one of them next to the other, then use Firefox's own Ctrl-Tab and Ctrl-Shift-Tab to hop back and forth (those are the Next Tab and Previous Tab keyboard shortcuts, respectively), but that's way less convenient.
With Focus Last Selected Tab installed, tapping Ctrl-. cycles you between the two most recently viewed tabs. As a blogger, I find this absolutely indispensable. Indeed, my brain is now permanently wired to use this shortcut -- one of many reasons I'll never move to Chrome or Internet Explorer.
By the way, there are actually several versions of FLST floating around. If you search for it on Mozilla's Add-ons site, you'll find the original -- which doesn't work in the latest versions of Firefox.
I'm partial to the one found at Gozer.org. You have to install the extension from that site (just click the Focus Last Selected Tab link midway down the page), but I can vouch for its safety.