The main problem (that also lead to the development of Decians Iceweasel) is the [trademark issue][1], that binds the use of the name Firefox and the logo to certain requirements, that were unacceptable to Debian and GNU. For instance in the case of Debian Mozilla demanded to upgrade the version of Firefox instead of backport bugfixes as Debian does it. These issues are avoided by renaming it and use a different logo. As the creation of [Icecat][2] shows, that is pretty easy and functionality is unaffected.

Moreover Icecat (and different distributions) remove a number of files from the distribution, they esteem as nonfree. Here is a [list of files removed from Firefox for Icecat][3].

And finally another issue the GNU-project has, is also bolded in your quotation. Firefox allows proprietary plugins and extensions. Moreover, these are distributed through Mozilla's infrastructure and are recommended in the process.


  [1]: https://en.wikipedia.org/wiki/Mozilla_Corporation_software_rebranded_by_the_Debian_project
  [2]: https://en.wikipedia.org/wiki/GNU_IceCat
  [3]: http://cvs.savannah.gnu.org/viewvc/gnuzilla/remove.nonfree?revision=1.9&root=gnuzilla&view=markup