Iceweasel is Debian's fork of Mozilla Firefox. They forked it in order to conform with their free software guidelines, and to satisfy the Mozilla Foundation's requests.

Now, when hearing that Debian had to fork Firefox because Firefox wasn't satisfying their free software guidelines, most people have to do a double take at the very least, or will fire vitriol at Debian at realising that they heard it right: Debian doesn't think that Firefox is free enough, so they forked it. Debian demonstrates everything that's wrong with free software zealotry; Debian needs to get off its high horse; Debian is hurting all others who want to make free software; Debian this, Debian that, blah, blah, blah. Then there's also the other extreme, zomg, Mozilla is teh evil; it won't allow completely free usage of its software; this is what happens when free software comes from defunct corporations; sell outs, blah, blah, blah.

In actuality, the drama is much less than that. What happens is that Debian likes to tinker with its software, and often will modify software that it nabs from around the world (the software from around the world is called "the upstream source" in Debianspeak), fix it, patch it, modify it, and often even send these modifications back upstream. They did this with Mozilla Firefox, but Mozilla doesn't accept modifications just like that. Now, since Firefox is after all free software (it's also open source, but that's besides the point), Debian is in its right to modify and distribute Firefox as it sees fit. The one caveat is that although Firefox and its source itself is free, its logo never was, although its name was until recently, middle 2006.

What Mozilla used to request is that modified Firefoxes couldn't carry the characteristic red-fox-humping-the-Earth logo and instead demanded that an unhumped Earth without the fox be used as the logo. In addition, modified Firefoxes had to be termed "community editions", and this is exactly the path that the OpenBSD project has followed with its Firefox: they modified it, removed the fox humping, and labelled it Community Edition. Debian followed a similar path except for not labelling it Community Edition, and Mozilla said this was fine too, until recently.

Then in February of 2006, Mike Connor acting as representative of the Mozilla Foundation told Debian that Mozilla no longer liked the way Debian was using the Firefox software. They had two choices: either use the official binaries provided by Mozilla, or remove any mention of being Firefox from their software. Debian wasn't happy with the Mozilla binaries because they contained bugs that Debian had already fixed but that the Mozilla development team wouldn't adopt (they wouldn't adopt them mostly because they referred to older versions of the software and the Mozilla team works only on the latest version of Firefox). Debian also likes to keep older versions of software around for their stable distribution, hence the interest of patching their own older versions of Firefox well after the Mozilla Foundation has stopped supporting older versions. A further problem was that if they kept the Mozilla binaries, then they couldn't modify them, contrary to what is explictly expressed in Debian's Free Software Guideline nĂșmero tres:

Derived Works

The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.

Thus Debian opted for choice number two: they removed all Firefox branding from Firefox, renamed the term to Iceweasel after a Matt Groening quote from Life in Hell, and they came up with their own logo, a green blue network-looking sphere humped by a white furry creature (presumably the aforementioned Iceweasel humping the internet). Thus Debian gets to keep its own patches and security fixes, and at the same time Mozilla gets to keep its brand recognition without interference from Debian. Thunderbird and Seamonkey had to undergo similar treatment and became Icedove and Iceape respectively.

Iceweasel is based on version 2.0.* of Firefox, and save for the name and who humps whom, it is identical to Firefox as far as I can tell down to the very same legendary memory leak that has been unfixed for years in Firefox (although I understand that there are minor fixes under the hood).

There are still a few traces of Firefoxness branding here and there, such as the page from The Book of Mozilla still hasn't changed (perhaps, "And Lo, The Father saith, thou shalt not fly in my name, whence the heat grew cold and still the followers of Mammon trembled as Fire and Ice together may crush their souls" would be a suitable replacement?), and here and there a few other files still have the Firefox name in them, although presumably those will also be eventually removed.

So it's not a big deal, and Debianistas aren't all a bunch of religious zealot nuts (except perhaps for myself), they are simply adhering to free software principles and playing friendly with the Mozilla Foundation who is asking its brand to be respected, and Mozilla isn't a heartless baby-eating company just because they are requesting brand name recognition. As for the rest of us, we still get our software, and we can still use it for whatever purpose and poke around its internals if we so feel like it.