KaiOS got Whatsapp support thanks to shipping in India with a single carrier (Jio) that has a very large user base. Deployment in the rest of the world has been a struggle and the company is not in great shape.
All that to say that Mozilla could have kept the lights on for a couple more years and get access to large markets. Hard to predict what would have happened but we certainly would have more diversity in the OS space.
Which would have been because they thought they could make more money using it than Android.
Which probably wouldn't have bode well for user-friendly changes to the base image.
Android's value prop to manufacturers was "Was to sell a lot of mobile phones, but not have to pay for most of the development? And get a working Maps solution? Here you go." Which Google could afford to torch money on.
And I'd imagine carriers don't, as they'd no doubt love to go back to the feature-phone days, but they're all (individually) too weak to do anything about it.
Only aggregated can they offer the resources to support an alternative.
But Mozilla had no chance in the real smartphone market. If Microsoft couldn't manage to attract developers with their billions and dedicated hardware, Firefox supplying only the OS and no hardware just had zero chance to make it mainstream. It would have been relegated to the same position as Sailfish: A cool curiosity but not interesting enough for anyone but some hobbyists to develop for.
I don't think it was a bad idea trying: At that time the duopoly in the smartphone market was not as firmly established and there were other open projects like Ubuntu as well. They might have attracted a huge party like Samsung (after all, they did go for Tizen in the end!) and things might have worked out differently. But the choice to drop it was inevitable at that point.