FirefoxOS was also exclusively targeting the touchscreen market, which by that point was already totally saturated. Their partner deals weren't the best either, and they really launched their product before it was ready which sullied their image quite a bit.

KaiOS had the benefit of hindsight to see where Mozilla fell over, put in mitigations, and then shift focus to non-touchscreen devices to lower costs even further.

A lot of their stack is still open source under the MPLv2, however there are rumours that Google is 'encouraging' KaiOS to shift their rendering engine over to Blink. Whether this happens or not remains to be seen.