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?
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.