One significant point about debian and the DFSG in practice is that the debian project maintainers understand that code is data and data is code. This means that, for a package to meet the DFSG guidelines, not only must the program source be available, redistributable in modified form, but the associated media (pictures, documentation, et cetera) must be redistributable in modified form.
For a long time, this meant Firefox was restricted to the non-free repo, since Mozilla prohibited redistribution of the name and icon on source compiled versions, so debian’s version was branded Iceweasel (similar to the chrome/chromium split).