It's a bit of a pain to explain why Firefox is the same thing as Ice Weasel.
"Because the Mozilla people have done something insane, egotistical, and counter-productive." There, that wasn't very painful.
As you imply, the reason that anyone who forks Firefox should be able to call the binaries their project produces firefox is the same reason that anyone who forks ls should be able to call the binary their project produces ls. Anything else just causes user confusion. If you want to have a trademark because you think you're so darn good that anyone else's version of your code just won't be the same, trademark your organisation name, not your project name. FSF Emacs vs XEmacs, FSF Glibc vs eglibc, coreutils ls vs BSD ls: there's no ambiguity in any of those cases.
By trademarking the name of the product, and imposing restrictions on uses of that trademark which affect people carrying out the normal processes of open-source natural selection, the Mozilla people have placed their own personal egos above the quality of the user experience. That's the sort of behaviour that open-source more usually comes as a blessed relief from.
That such restrictions on trademarked product names are allegedly compatible with open-source licences should be viewed as a bug in those licences, and Debian are absolutely right to call the Mozilla people on not being DFSG-free. If you want to pinpoint the moment they lost their way, it was when they trademarked "Mozilla" and named their foundation after it, and changed the mozilla tree so that it no longer built a binary called mozilla. If when they realised their "mistake" they'd changed the foundation name not the binary name, there'd be the Seamonkey Foundation's mozilla versus everyone else's mozilla, which would be perfectly straightforward, and they'd have generated much less ill will.