Iceweasel is a fork of Firefox which was made by the Debian project because the license conditions of the Firefox name and logo violate the Debian Free Software Guidelines.Did Debian do any other changes to Firefox which were not just cosmetic?
Differences between Firefox and Iceweasel
forking;dfsg
In Debian, Iceweasel is Firefox once again, since March 10, 2016. There's an Iceweasel branding add-on which can be used to restore the Iceweasel branding.There are some non-cosmetic differences between Mozilla's version and Debian's; you can see all the patches here. You'll find changes which disable phone-home features (Firefox health report), change the way plugins and extensions are handled to work better in the context of a distribution with packaged plugins and extensions... There are also a series of bug fixes backported from work-in-progress upstream versions.These are the kinds of changes you could find in any package in Debian (the old branding changes weren't, of course, although quite a few Debian packages include minor branding changes  e.g. adding a Debian mention in version numbers in gcc and Wine). The Chromium package for example also disables various phone-home features; many packages include backported bug fixes, and changes to integrate the software into the distribution.