What legal reasons exactly? It's called Firefox on all the other platforms, yes? Is it because they don't want to contribute back their own complete sources to this *open source* project?
Actually no, Debian also contains a version of Firefox that's called IceWeasel for legal reasons, and that's an open-source project. Something about all modifications having to be approved by the Mozilla Foundation (including those necessary for porting).
Now a quick word about *open source*; not all open source licenses are GPL, and if the license says that the code can be used in closed-source projects, then the code can be used in closed-source projects (provided that all requirements are met). The MPL license clearly says that the code can be used together with closed-source files, so that means that they clearly have the original author's permission to do so.
Hans