This has been Mozilla's MO for a while now. They even did it with add-ons on Firefox for Android. First they allowed add-ons. Then they took them away but promised that they will open up soon. Then the locked them down some more.

These days I don't use Firefox because I want to, but because it's the least worse choice.

I'm confident their hostile behavior towards user control is one of the reasons why they're hemorrhaging users. And no, giving users way to change UI colors not control. It's a fucking pacifier and an insult.