system
A chat on subreddit /r/pcmasterrace
human
That was a very serious thing btw. The Linux distribution in question (Debian) made changes to Firefox to better integrate it, thereby violating the Firefox trademark, so they had to rename it.

Firefox and Debian later agreed on what changes were acceptable so these days Firefox is Firefox again.
gpt
The way I saw it was the problem had to do with the fact Debian has always been a very "purist" distro in regards to open-source licensing. They've always been very opposed to distributing anything that isn't completely and totally free. Mozilla's MPL license is open source, but has some restrictions. 

Iceweasel was missing a few features that were included in Firefox because they weren't open-source enough. It's like chrome vs chromium, or Android versus AOSP. Chrome and Android aren't open source, they're built on open-source projects that have proprietary bits tacked on like pdf viewers or google translate integration or the play store, etc.

Debian's idea of what's "proprietary" is way more strict than other distros. This is a big part of why Ubuntu became so popular. It didn't care about licensing, which meant your video card magically worked without having to deal with compiling kernel modules.