Firefox 57 or later does not support traditional add-on technology so we will not be able to rebrand the entire browser using add-ons.
Technically, a new Debian (or any other distribution) package might be able to do that. Some may be interested in this possibility. Contact the developer for details.
Debian GNU/Linux has distributed Iceweasel, a rebranded version of Firefox, due to licensing uncertainties. But now with all problems resolved, Iceweasel is gone! This extension restores Iceweasel branding (partially) for Iceweasel Lovers. Don't publicly distribute this extension packaged with official Firefox unless you're absolutely sure that it is legal for you!
Original copyright 2005-2014, Mike Hommey
The artwork is copyright Unicko.
Complete Debian package with desktop icons
[Debian] Support user choice!