Nobody is "prohibited" from doing so. You want a Firefox that lets you do whatever you want, you can easily use an unbranded or pre-release build, or even roll your own or use someone else's lightly-tweaked fork. Nobody owes you an officially supported product that's for a wide audience and is also a complete free for all. Even Ubuntu and other Linux distros require you to opt into third-party software channels. You just don't like the specific choice that Mozilla is giving you, but it's still very easily there.