Our code is free, but we do strictly enforce our trademark rights, we must, in order to keep them valid. Our trademarks include, among others, the names Mozilla, Firefox, Thunderbird, Bugzilla and XUL, as well as the Mozilla logo, Firefox logo, Thunderbird logo and the red lizard logo. This means that, while you have considerable freedom to redistribute and modify our software, there are tight restrictions on your ability to use the Mozilla name and logos, even when built into binaries that we provide.
Basically, you are allowed to change/use their code, but you can not release it using the Mozilla name. The logic behind this is, if joe developer releases his own version of Firefox that is not of high quality, Mozilla will get "a bad name". Now enters Debian. Debian uses Mozilla code, and modifies it to include in their operating system. Debian is known for their quality software, so Mozilla waves their normal policy specifically, and only for Debian. However, DFSG says in article 8
License Must Not Be Specific to Debian. The rights attached to the program must not depend on the program''s being part of a Debian system. If the program is extracted from Debian and used or distributed without Debian but otherwise within the terms of the program''s license, all parties to whom the program is redistributed should have the same rights as those that are granted in conjunction with the Debian system.
This says that if Debian gets certain rights, those rights must be passed on to everyone using Debian. If Debian uses the Mozilla trademark and doesn''t pass it on, Debian is violating its own guidelines. If Debian doesn''t use the Mozilla trademark, they will have to name it something different and it will be a splinter in the FOSS community. To those whom ask "why does this matter", if Debian were to agree to this Knoppix, Mepis, Ubuntu, and every other Debian based operating system could not use the Mozilla trademark. Lets hope Mozilla comes to develop a trademark policy that is in-line with the concept of freedom.