The average user isn’t using Firefox to begin with. In fact 5% market share for Mozilla is probably pretty generous.

I agree with you that Mozilla has certainly dropped the ball regarding this change. Much like pocket, the Mr Robot ARG, the Quantum update, DoH with Cloudflare as the default provider, and so on. But again all but the Quantum update have been user configurable, and in my experience, have not been reverted unlike my browser preferences on Windows, where Edge hijacks anything and everything with very little recourse.