Firefox is developed by a group of members and is fully Open-Source. Although Internet Explorer is free (as such) it doesn't fall into Open-Source as its source code is not distributed and it is develpped entirely by Microsoft.
As such, Firefox posseses many powerful features that IE is still lacking such as Tabbed-browsing, pop-up blocks (which were implemented long before MS ever concieved the idea) & much more. Firefox is also seen to be a 'safer' and more 'secure' browser as it is in fact the hackers themselves which are probably develping the thing.

As a whole, people usually have their own preference and web designers will usually design for Internet Explorer and do their post-testing on Firefox and Netscape.

Hope that helps,
~Equinox