I've been in commercial contact with FirefoxOS people, and they were definitely targeting phones that are the same specs as what KaiOS currently supports. (512MB RAM, 4GB storage, which is half the RAM and half the storage of Android Go)
I don't really know they were calling it smartphones or feature phones though, that might change things.
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.
Mozilla started a feature phone project, that was later canceled. One issue at the time was that the carrier partner was Verizon, and that was a huge gap of values due to how VZ was fighting against Net Neutrality.
Switching to Blink would be a huge undertaking, ditto for moving off classic Gecko.
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).
For starters, they targeted feature phones instead of Smartphones