So, imagine you have a toy car that looks like Lightning McQueen from the movie Cars. That toy car is like a brand. People recognize it as being Lightning McQueen. Now, imagine if someone else starts making toy cars that look exactly like Lightning McQueen, but they call their toy car something else, like "Fast Car." Would that be fair?
Well, that's kind of what happened with Debian and Mozilla. Mozilla is the company that makes the Firefox web browser. They have a trademark on the name "Firefox," which means no one else is supposed to use that name for their own browser. But Debian, which is a type of computer operating system that some people use, didn't want to use the name "Firefox." They wanted to call their version of the browser "Iceweasel."
Mozilla didn't like that because they didn't want people to get confused and think that "Iceweasel" was the same thing as "Firefox." They also didn't want "Iceweasel" to have any of the Mozilla logos on it. But Debian thought that was unfair since they were still using the same code as Firefox, just with a different name and logo.
So, there was a big argument between Debian and Mozilla about whether or not Debian could use the name "Iceweasel" and the Mozilla logos. In the end, they worked out a compromise where Debian could use the code from Firefox, but they had to change the name and the logos so that it was clear it wasn't the same thing as Firefox. And that's why some people use "Iceweasel" instead of "Firefox" on their Debian system.