Here's a brief Q&A by Christopher Beard with Mozillas perspective on the use of Firefox in Debian and Ubuntu. Mark Shuttleworth now also comments it in his blog.
Read it here
I understand that Ubuntu is based upon Debian. Is that the same or different than the IceWeasel browser that Debian is shipping with their latest release?
It's different. The patches that Debian applied to the Mozilla source code (which then resulted in their IceWeasel product) are more significant in scope than those in what Ubuntu is shipping (and branding as an official Mozilla Firefox release). Firefox in Ubuntu represents a somewhat more modest set of divergences from original Mozilla source code.
Its a very important thing to know that an inability to agree on something – even if that thing turns out to be a dealbreaker – doesnt mean that the other person is a bad person. Give credit where it is due, state your differences simply and without prejudice. Debian and Mozilla should be able to work together effectively on a browser, even if they cant agree on a way to call it Firefox.