Google has enough cash to invest into both native and web platforms, and see which one (if any) prevails. At the end of the day, more platforms is access to more developers and the apps they make, which in turn is more incentive for end users to buy the devices, whatever OS they happen to run.

Mozilla is leveraging a large existing developer base and relying on the brand power and capabilities of Firefox that they have full control over. Their choice of market is interesting too, as it seems to be largely served by low-cost hardware stuck with outdated Android installs with no upgrade paths on the horizon. Meanwhile, Google is distancing itself from Android as a product and instead pushing own devices as "the" Android experience, leaving the low-cost hardware market with stale software.

These seem like thought-out strategic moves Mozilla is capable of pulling. It'll be interesting to see how things play out.