If you really want to go into the gritty details - in 2005, Gervase Markham, representing himself as speaking on behalf of Mozilla in this matter, indicated that what Debian was doing was okay:
(Also see the next few messages from Eric and Mike where they worked out some of the details)
So the short version is that there was a lot of confusion from Mozilla about what Debian should be doing here, and at each step Debian adjusted the contents of the package in accordance with the information given.
As far as the artwork license is concerned, that's actually a distraction; the conclusion of the discussion with Mike Connor was that Debian could only use the firefox trademark if the Mozilla Foundation was pre-approving all releases, and it would be quite tricky to make this work with Debian's release processes. Using the iceweasel branding is much less work for everybody and doesn't seem to cause any problems for users.
https://lists.debian.org/debian-devel/2005/06/msg01182.html
In 2006, Mike Connor contacted Debian and stopped just short of saying outright that Gervase should never have done that:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=354622#59
(Also see the next few messages from Eric and Mike where they worked out some of the details)
So the short version is that there was a lot of confusion from Mozilla about what Debian should be doing here, and at each step Debian adjusted the contents of the package in accordance with the information given.
As far as the artwork license is concerned, that's actually a distraction; the conclusion of the discussion with Mike Connor was that Debian could only use the firefox trademark if the Mozilla Foundation was pre-approving all releases, and it would be quite tricky to make this work with Debian's release processes. Using the iceweasel branding is much less work for everybody and doesn't seem to cause any problems for users.