Frankly, that doesn't mean much; the MPL allows one to freely mix open and proprietary files on the same project. Sure, they can't literally close Mozilla's code, but they can still make their forked project mostly proprietary.
[1] almost, because every smartphone needs proprietary drivers to run …
Free phones are slowly popping up https://puri.sm/products/librem-5/
It’s bizarre to me when people complain about things like “tivoization” or “open core”. These are by design or they are solved problems if the creators care for the trade offs.
That is the Mozilla community’s issue. They chose the licensing. They have revised the strategy a number of times. I remember when code was tri-licensed.
Or it’s also a consumers issue. Money talks.