I think the engine is open source; but the real appeal is using the real time collab online tool, which is proprietary and had the drawbacks you mention.
PlayCanvas was started by ex-EA/Activision/Sony devs. But Mozilla partnered with PlayCanvas to open source the engine and later launch Firefox's WebGL 2 support with the After the Flood demo: http://aftertheflood.playcanvas.com/
Their particular IDE may be closed-source, but the engine is not tied to it.
IIRC playcanvas was started by ex-mozilla people.
I played around with the engine a while ago and was quite impressed by the all-in-one-ness of it.