It has been good at improving Firefox, but if you look at the wider picture then you see that Firefox has been falling in usage and struggling to keep up with webkit/blink. And IMO a large part of that is because core parts of Firefox Gecko and Spidermonkey are much less widely used (by as many apps/companies) than equivalents like Webkit/Blink/V8/JSC, and this because they are not easily embeddedable and their codebases are harder to work with.

From this perspective, not focussing on documentation and making components usable externally is pretty short sighted.