It's notable that there's no real nodejs equivalent running on Mozilla tech. I'd love for someone closer to the tech to explain why there's not a rich ecosystem behind spidermonkey, etc.
I too would love to learn more about that.
I am not sure about the current state. But "back then" all the components in Firefox were tightly coupled and almost impossible to extract on their own.
"Back then" being, IIRC, 2012 or so, when I briefly worked on the web and CMS side of a project that used HTML + CSS (and a tiny bit of JS) to render the UI of a media-box. The OS was basically a thing that could boot a "browser" and handle network stuff. Firefox was not an option, as it was near impossible to even remove things like the address bar, tab handling and all that. But the hardware was so underpowered, that a full browser was not an option. Yet "yet another khtml" wrapped in the most basic "executable" did just fine.
But this is a while ago, and only one project that chose not to use Firefox/gecko.