KaiOS takes a significantly different approach in bringing FirefoxOS to market than Mozilla did.
For starters, they targeted feature phones instead of Smartphones
For starters, they targeted feature phones instead of Smartphones
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.
Mozilla ditched Firefox OS & Thunderbird over the huge amount of work that would need to be redone to move to newer Gecko (as Servo components were ported over and swapped in via Project Quantum), but that would likely be less work than getting Blink to be responsive with minuscule amounts of RAM, then go app by app and optimize the crap out of it (which Mozilla had dozens to hundreds of devs doing for Firefox OS while they were developing it).