TL;DR: Iceweasel was rebranded back to Firefox in Feb '16. You can no longer install Iceweasel.
Firefox is open source. This means that anyone can freely distribute and modify the source code.
However, the laws surrounding the trademark 'Firefox' and the Firefox logo are slightly different. The trademark and logo may only be used with unmodified/official versions of the source code. (This is because Mozilla wants users to have a consistent experience when using 'Firefox')
Debian (the Linux distro which Raspbian is based off) has a set of guidelines called the Debian Free Software Guidelines, which determines what software is allowed to be included in Debian.
Because the restrictions placed on the logo and trademark were not compatible with the guidelines, the distribution of Firefox to be included in Debian had to be rebranded. After many debates etc, the name Iceweasel was born and Firefox was rebranded.
However, in February 2016, Mozilla allowed Debian to use the Firefox trademark and logos in their distributions:
Mozilla recognizes that patches applied to Iceweasel/Firefox don't
impact the quality of the product.
In case of derivatives of Debian, Firefox branding can be used as long
as the patches applied are in the same category as described above.
Hence, Iceweasel was 'un-rebranded' back to Firefox.
So why can't I install Iceweasel?
Because Iceweasel has been rebranded to Firefox, the Iceweasel package has been discontinued, and instead Firefox is automatically installed. You can no longer install Iceweasel.