It’s really not ambiguous. Anyone can fork Firefox, make any changes they want, and release it with different branding. This is the goal of open source.
The term you’re looking for is free software. By making this change Firefox is no longer respecting the freedom of their users. That’s the “F” in FOSS. It’s possible for Firefox to remain open source without being free software.
“How am I meant to hit you?”
-A person who has just hit someone...twice.