"Mozilla's beef is with Debian or anybody else messing around with code or the settings and still trying to palm it off as Mozilla Firefox. People are still free to branch the code and call it anything they like, which is just what Debian has done."
What if the Linux kernel developers did the same thing - the only way you can call it Linux is to distribute an official kernel.org release with no custom patches? KDE? GNOME? vim? emacs? every single application you use? It would really suck to install software, because there would be no name you could use to identify what your distribution was shipping without infringing on someone else's trademark.
Sure, trademark law allows Mozilla to do this, but it's been custom in the free software community *not* to do this for over 25 years.
The GPL even includes a specific provision for Mozilla's worry of "someone will patch it to break it and our reputation will be tarnished" - if you distribute a modified copy, "the work must carry prominent notices stating that you modified it, and giving a relevant date".