Firefox IS Better

I used to be a dedicated IE (Microsoft Internet Explorer) guy. I liked the way it worked. Recently, though, I've fully switched over to Firefox. I've had Firefox installed before, but I still used IE as my default browser, until one day I saw the light. I imported all my favorites and settings into Firefox, and set it as my default browser. I have no regrets about this. Firefox just is the better browser, for many reasons.

For Regular People

Firefox is faster and more secure than other browsers. Tests have been done to prove this. During some of these experiments, IE crashed so hard that it couldn't even complete the test.
There are also so many things you can do to customize it to make it do everything the way you want it to. From around the net you can find Add-ons and Skins for Firefox to change or add to the power or just to change the look. To tell you something funny, I've actually found a few skins that make Firefox look exactly like IE (myFireFox 2.0.0.10), for those who are going to miss it.
Firefox also comes built in with a spell-checker. This is when you're typing Ramblings, for example, it underlines the words you got wrong in red, same as Word, and gives you choices of the correct spelling. This also comes paired with auto completing. Firefox detects fields such as "Email:" and "Name:" and lets you know that you can let Firefox fill those in for you.

For Web Designers

There are some great tools out there that you stick onto Firefox as Add-ons to aid you in your web development. I know that if there was a problem in my code and I tried the page with IE, it would tell me so. In Firefox it would actually tell me the type of problem (whenever possible) and the line in the code that it occurs in. This is one of the many benefits of the 2 add-ons I have installed: Firebug and Web Developer Toolbar.

For Everybody

Firefox now is the most popular browser used in the world. I can partially prove this by what I see through Google Analytics. More than 65% of the visitors to my websites use Firefox, while the remaining 35% use other browsers including IE. After some time of using Firefox steadily, I do start noticing how much faster it is, and the greater ease of browsing the World Wide Web.
Tabbed browsing is also done better in Firefox. You can even get an Add-on to make it even more better-der-er. I've done it and it's superb. One part of Firefox tabs is what I enjoy the most. Do you even accidentally close a tab? We all do, but in Firefox you have a sub-section in the history where you can recover "recently closed tabs". Them Firefox developers are amazing!
For more information, you can always do a search on the "benefits of Firefox". You'll be amazed at all the good things you find in the results.

Recommended Add-ons

There are probably many more that I haven't discovered yet, but I'm sure we'll all hear about it in the comments, along with your recommendations and experiences with Firefox.

Speeding up Firefox:

Check out Cutting Up a Dog on Fire: a guide to hacking Firefox