Why I use FireFox

Because it's beautiful. It's free and open-sourced. It also has 3rd party add-ons.
I used to have "Ad block Plus" which did what it said. It blocked every ad. No extra load time for ads, no ads period. I did have to disable it, because it interfered with some things I need to see as a web developer and flash animator, but just because it had some (minor) problems doesn't mean that it isn't great. It was just my preference to turn it off.
Now, I have an add-on called "Web Developer" which is so awesome I can't describe it to justice. It allows you to view website architecture, elements, tags, usage, links, CSS and a hundred other options right on screen depending on what you select to view. It will validate and analyze websites for size, load ability, link validity, and even let you rewrite the HTML of the page you are on (for your browser only), save it or revert to the web version.
Indispensable. Incomparable. Fn Free.