Because Debian issues a version of Firefox that contains
    unofficial patches, they cannot legally call their
    distribution "Firefox", since Mozilla hasn't officially
    blessed that exact codebase.

There's a lot of sense to this. Consider the hacked up configurations of vim that ship with redhat and debian. The maintainers code intrusive personal-favourite settings into /etc/vimrc (e.g. settings that reformat your code). People get annoyed by the behaviour and think "vim sucks" whereas the default vim distro is conservative about intrusive behaviour.

Mozilla are happy for distros to put out their code - just not hacked up versions of it with the same name. Good for them.