In Linux - you should worry about your default browser, as well as you shoud worry about your favorite action movie. You cannot change either of them, but why the heck... Rambo is cool.
Since you know it isn't gonna change - unless you exciplitly tell it to do - the best thing to do is to create a /bin folder in your home directory. Then - since it will most definitely not work - the next thing is to replace the /usr/bin/firefox links, and redirect it to /usr/share/iron/iron
Although, since we're talking about Linux, this will not be working either. So in order to define your preferred browser, you'll have to move "/usr/bin/firefox" into another location, and replace it with a link to your preferred browser. In this case /usr/share/iron/iron.