Debian is a bastion of freedom with strict rules about what does or
does not qualify as freedom. We use debian for a variety of reasons,
but we all (should) know that at the core of Debian is this
freedom. We choose whether to use less free portions of Debian, but
are not forced to in any way as the core of Debian is free.
Firefox is the current front-runner for secure, stable, all-around
good web-browsers. This is not a judgement of FF, just an
observation. Mozilla has changed how it handles some of its IP and the
freedom of that IP (specifically the artwork). Okay, maybe they
haven't changed it, but they are trying to enforce it. Regardless,
this less than free IP is not acceptable to Debian. But, Debian wants
to include FF in its distribution for reasons listed above. So there
are two alternatives: either Moz relaxes its rules around the
less-than-free artwork, or Debian packages a more free version of FF