Yeah, but it'd be a silly requirement, and would make syncing upstream changes to your fork needlessly difficult. My point is the Firefox branding stuff does not apply to the source code of forks.
Venturebeat claims it's based on Chromium, but they also seem to be pretty confused because they're calling it "a brand new browser, because (builing on top of existing browsers) would imply limited functionality by virtue of their application programming interfaces (APIs) and less impressive performance gains"
Firefox on iOS is built with WebKit, not Gecko [1]. IIRC, it's because Apple doesn't like engines that are not WebKit. Chromium uses Blink, which is a fork of WebKit, so Firefox for iOS and Chromium are relatives in a sense.