Many browsers do provide add-on functionality. All the major browsers do: Internet Explorer, Firefox, Chrome, Opera, Safari, all come with some sort of add-on functionality.

I do use add-ons, and I do find them invaluable. In Firefox, Firebug more often than not is valuable in debugging pages and stuff. I find myself using it quite often if I feel the need to do so.

The only add-on I've ever used was called Fireshot which screen captured the entire page of a website that Firefox was on. I used it until I figured out the Print Screen key.

Unless you want to count themes as an add-on, because I've had plenty of different themes.

I'm always using add-ons, seems. It's what makes my Firefox browser what it is to me, especially with the Personas add-on that skins it. I'm currently using an Okami skin on my desktop while Zoroark on my netbook.

I use Firebug to do many silly (and sometimes unethical) things. I also have a Find-and-replace tool that frankly only has a few applications. Then there's Destroy the Web, which is pretty funny and don't use often at all. It turns every web page into a gallery shooter kind of game.