Mozilla is forcing Debian to stop using the Firefox name with the version of Firefox that they ship. Debian doesn't use the official Firefox logo because it is distributed under a non-Free copyright license, which violates Debian's Free Software Guidelines. For branding reasons, Mozilla wants any version of Firefox that uses the Firefox name to also use the official logo. Until recently, Debian had an agreement with Mozilla that allowed them to separate the logo and the name, but Mozilla has reversed that decision.

At the same time, the GNU Project has launched a completely Free version of Firefox called IceWeasel and it seems likely that Debian will ship IceWeasel instead of Firefox. Some folks on the Ubuntu forums have come up with some spiffy IceWeasel logos but there doesn't seem to be any indication as to how Ubuntu will deal with the Firefox naming issue.

The disagreement seems silly to me and harms everyone involved. It is strictly a branding issue for Mozilla and they could easily continue to allow the use of the Firefox name without the logo or re-license the logo under Free terms. Having numerous Linux distros shipping a browser called IceWeasel is going to be much worse for the Firefox brand than a few missing logos.

There is already a nice Wikipedia article on IceWeasel.