I corresponded with the maintainer of the iceweasel package for debian, and he said that you are right that the debian iceweasel is not the same as the gnuzilla iceweasel. But he also said that the debian iceweasel source was free for everyone to use. The correspondence is below (though it might be hard to read).
The page here: http://packages.debian.org/unstable/source/iceweasel
Has the source file, which is just the renamed firefox source, and then a .diff.gz file, which has some patches, the iceweasel icon, and some scripts for debian. He said you could review the changes in the file and decide what you want.
You can do the same with the icedove package, as far as I know (though that maintainer never wrote me back):http://packages.debian.org/unstable/source/icedove