From the "read the licences or go away" files:

While Mozilla's Firefox is an open source technology, the term Firefox itself is a trademarked term (by Mozilla).

As such for every version of Mozilla Firefox in binary form that is available to end-users (Windows, Mac, Linux) there is a EULA (end user license agreement) that outlines the terms. This is a seperate issue that open source code and unfortunately it's one that Ubuntu users (as opposed to every other Linux distribution) seem to be opposed too.

Mark Shuttleworth - leader of Ubuntu has publicly posted that the Mozilla EULA should stay though he's opposed to the idea of the EULA in general.

"I think it's perfectly reasonable for Mozilla to have requirements and guidelines for the use of their trademark - we have the same for Ubuntu, and many other free software projects do the same. I would in fact consider it a best practice to have a good brand on a free software project, which means having trademark guidelines," Shuttleworth wrote. "That said, I would not consider an EULA as a best practice. It's unfortunate that Mozilla feels this is absolutely necessary, but they do, and none of us are in a position to be experts about the legal constraints which Mozilla feels apply to them"

Frankly I'm confused by Shuttleworth's statements. He considers EULA's best pracitice when it serves Ubuntu's needs but then when it comes to Mozilla (in the same statement too..) he no longer consider it a best practice.
**UPDATED** Readers have correctly pointed out that Ubuntu does not have a formal EULA in the way that Mozilla does for Firefox.

From a practical point of view as an end-user - it doesn't really matter. The user gets to use the browser. For licensing zealots, there is always a choice you can use an unbranded version of Firefox (IceWeasel).

**UPDATED** I spoke with Mitchell Baker of Mozilla last night - read the full story on the main site.