firefox: enable official branding

As stated by Sylvestre Ledru (@sylvestre) on Nov 22, 2017 at
https://github.com/NixOS/nixpkgs/issues/31843#issuecomment-346372756 we
have permission to use the official firefox branding.

Fur purposes of documentation the statement of @sylvestre:
> As the person who did part of the work described in the LWN article
> and release manager working for Mozilla, I can confirm the statement
> that I made in
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=815006
>
> @garbas shared with me the list of patches applied for the Nix package.
> As they are just for portability and tiny modifications, they don't
> alter the experience of the product. In parallel, Rok also shared the
> build options. They seem good (even if I cannot judge the quality of the
> packaging of the underlying dependencies like sqlite, png, etc).
> Therefor, as long as you keep the patch queue sane and you don't alter
> the experience of Firefox users, you won't have any issues using the
> official branding.