GeckoView for Android
As the original author of GtkMozEmbed I'd like to say that this is way too little, way to late. Mozilla has historically been very much against anyone trying to use the engine in anything other than the firefox app.This is completely up to them of course, and it probably let them move firefox forward faster, due to not having to care about APIs. However, it also means that chrome completely took over the entire market with electron and similar things.