Nearly every time I meet with a representative from one of the Web browsers, the questions of "Which browser do you use?" and "Why?" inevitably come up. The answer to the first is still Mozilla Firefox, like most people I know. The short answer to the second is "extensions," which is an easier way of saying "more functionality and personal customization."
The follow-up question is invariably, "What's your favorite extension?" and that's much harder, because it's not any one particular extension that makes Firefox great--it's the ability to mix and match the add-ons that you want to use, tweak the options for each particular extension, and create a final browser that works exactly the way you want. If there's no add-on for the feature you want, you can always build your own.
Pressed for a response, however, I generally answer Tab Mix Plus. And when I think about it, it's true. I'll admit I can't imagine Firefox without AdBlock Plus, and the only toolbar I use is Web Developer, which is completely awesome. Some users swear by FoxyTunes, and it's the most popular extension on CNET Download.com, but I have no problem switching applications to control music playback.
It's true; the only extension I couldn't live without is Tab Mix Plus. It slices, it dices, it makes all of my tabbed browsing dreams come true.
What's really the best Firefox extension ever? Tell me about your favorite in the comments.