I had been using Google Chrome as my web browser until I decided to finally try firefox. 26.8% of all internet users are firefox users according to Wikipedia with only 17.6% using Chrome. I figured that more users could equal more extension support, better ease of use, and more tools than the minimalistic style browser that Chrome is. I was correct.
One of my favorite ideas I saw in firefox were the release channels for beta, current, and alpha. When firefox 5 was released there were already users testing firefox 9, ensuring that the release would mostly be swept of all bugs. You can choose what browser version you want in Linux by added the PPA for whatever version of Firefox you want. The most recent version will gain priority over ones before it. For instance, if you added the Beta PPA and then the Alpha PPA you would be put on the Alpha update channel. You can switch channels at any time by removing the PPA, but I would not recommend that if you are more than a few versions ahead. You could go months without any updates.
One of the two downsides to firefox that I noticed was speed. Firefox (version 6) is considerably slower than Google Chrome. I have slow internet speed, so any speed counts. But for anyone that has a high speed connection it should barely be a noticeable difference.
The other issue was the way tabs are run. In Google Chrome every tab is run as a new process, which people complain can eat away at RAM. But it is a wonderful way to do things, because one catastrophic tab failure will not bring down the entire web browser. While it is rare for this to happen in Firefox it has the ability to happen, which is more than enough to scare me when I am doing something critical through the web browser. Firefox makes up for it by saving tabs for opening the next time firefox opens after an unexpected shutdown.
Extensions are pieces of software created by firefox users that either add functionality or change the way something in the browser or on the web works. Google Chrome has a wide range of extensions, but not nearly as vast as firefox's collection. Some of the extensions are Operating System specific, so Linux / OS X users could be left out.
Some of the more popular extensions are Greasemonkey, Adblock, Firebug, and Speed dial
Greasemonkey allows scripts to be installed and managed. Scripts act similarly to extensions but affect only the way websites work. For a huge collection of scripts visit userscripts.org.
Adblock does exactly what the name makes you think it would do. It blocks ads. Not only does it block them, but it stops them from downloading; saving bandwidth. I am not an adblock user because ads support some of people's favorite websites like hotmail, facebook, and even Blogger.
Firebug is a webpage analyzer.
Speed dial is like the new tab page from Google Chrome, but they are only websites that you set. It has up to 9 saved pages per group and you have an unlimited amount of groups to make.
Google Chrome / Chromium should be the choice for anyone looking for speed, ease of use, and compatibility. When it is a match between Chrome and Firefox it mostly comes down to preference and how much addon support you want.