Sure - let them squeeze out the last drop of their one and only cash cow before it becomes utterly irrelevant - instead of investing in it and making it a core pillar of their strategy.
>See how Wikimedia Foundation is basically flush with cash,
Mozilla was (is?) flush with cash. Browser market share and corresponding user eyeballs are immensely valuable. That money isn't going back into Firefox development. This is why it is borderline criminal that they choose to focus on everything except Firefox.
Alternatively, what are they without Firefox?