It's probably too late now... but IMO, what should have been Mozilla's most natural progression towards financial security would have been with Thunderbird. Basically, they were in a position to offer what Outlook/Office365 email does a couple decades ago.
Integrate better calendar and contact management, then create a best of class commercial email service platform, and commercial hosted services around that.
I thought it would have been a great option long before Gmail was even a thought. Even today, they could work with or create a service like Protonmail or another system to offer these services. 10m users at $4/mo/user is $480m/year and that wouldn't be an unreasonable expectation just for the US market in 3-5 years given where they were in 2008.
Of course, I had similar thoughts about Blackberry when iOS and Android hit the market... since they were already entrenched in corporate email at a lot of places, they could have created a best in breed mobile client for their integrated usage ahead of MS playing catch up.
But with Mozilla, it would have been a natural extension as a commercial offering without disrupting the good will behind Firefox and Thunderbird as they were...