Iceweasel is a fork Mozilla Firefox[1] with the following purpose :
1. backporting of security fixes to declared Debian stable version.
2. no inclusion of trademarked Mozilla artwork (because of #1 above)
Beyond that, they will be basically identical. (quoting Roberto C. Sanchez post in debian-devel mailing list)

Iceweasel’s (and Firefox’s) latest version, 4.0, was recently released[2], on Mach 22nd, and i’ve just upgraded Iceweasel 3.6 to Iceweasel 4.0 on my Debian Wheezy.

If you’d like to upgrade yours, follow the steps bellow.

1. Add Mozilla’s repository to sources.list
Edit the /etc/apt/sources.list file and add the repository’s address:

Even though it’s Squeeze’s repository, it worked fine on my Wheezy.
You can type the following command to add it:

2. Update packages list

3. Install Iceweasel 4
You can check the version of iceweasel with the command bellow:

Install it with the following command:

Wait for it to finish and happy surfing!

Links/Sources:
[0] http://wiki.debian.org/Iceweasel
[1] http://mozilla.com/firefox
[2] http://blog.mozilla.com/blog/2011/03/22/mozilla-launches-firefox-4-and-delivers-a-fast-sleek-and-customizable-browsing-experience-to-more-than-400-million-users-worldwide-2/
[3] http://mozilla.debian.net/

FireFox 3.5, named ShiretokoShock, is out!

Get it at: http://bit.ly/ShiretokoShock

See also the Shiretoko Shock Campaign: http://www.spreadfirefox.com/shiretokoshock-campaign