The short answer is "It's all about personal preference". I personally use it until I am forced to use another, which these days is a rarity given one has the right extensions added. Some may argue that it is a pain to install extensions and fight through the newer versions of Firefox being incompatible with the extensions that are already installed.
I however find searching for and maintaining components to be part of the experience because I feel like I have control over the whole scope of browsing the web. Similarly, other browsers use tabbed browsing, integrated search, live RSS feeds and so on, but none of which I have found so far to be so heavily supported by peers.
Then there is the whole debate about security. All I have to say is that since switching to the fox, my browser hasn't been hijacked once, but not everyone will see these results. IE is made to run pretty much out of the box, besides the installation of some Media Helpers, and the occasional blunderous Active-X. One could ramble for hours, but I will just leave it at that.
As far as the Web Developer is concerned, Firefox is a must have as far as I am concerned. There are so many different toys that I sometimes forget that I even have them, although this is a rarity, because when a certain feature is missing, I notice. The search capabilities alone for me make it tops, they are second to none in my humble opinion.
I would keep going, but I could definately see where this would start looking as a bias post, so if there is anything specific you were wondering about, feel free to ask away.
Hope this helps,