You do know that most browsers let you change the User-Agent string to whatever you like?
This won't keep out IceWeasel users who actually understand the machine they are operating.
And motivations that you don't understand are not necessarily stupid. If you don't understand why it was important enough to the debian core team to bother with the effort of maintaining a forked version version of the browser; you are not able to comment intelligently on the topic.
Which is a fundamentally stupid thing to do.