I do not have an inside look, but if I remember correctly there were few calls about Mozilla losing focus on Firefox through making Firefox OS for a few years, which is exactly when the other vendors were reinventing their engines. They made some great work with Rust though (;
Firefox OS led Mozilla to push a lot on the performance front in Gecko, mostly in gfx land. The JS perf is mostly SpiderMonkey and there was very little FxOS specific work done (as far as I remember, we mostly got help to tune the GC heuristics) because the JS team was already streched pretty thin.
These days Moco doesn't even try on mobile anymore (hint, there is no performance testing at all on arm in https://arewefastyet.com/#).
The desktop Firefox front-end didn't evolve so much during the FxOS days but these was not the same team either... and you would notice that key people leading Fx front end have changed since.
