I see. Are there any major differences in the two licences? I'm interested why the OP specifically mentioned GPL forks of Firefox.

It's GPL compatible, and I wrote GPL instead of GNU. There's IceWeasel which is a Debian fork of Firefox to debrand it from all the Firefox trademarks and logos.

See:

https://en.wikipedia.org/wiki/GNU_IceCat

They basically use scripts that build IceCat apparently from the original source code. I'm guessing they diff against FireFox.