Summary: Initial implementation of OffscreenCanvas for WebGL and BitmapRenderer. This allows them to be used on a worker thread.
Preference: gfx.offscreencanvas.enabled, currently disabled by default.
Other browsers: Chrome has shipped it since 69.
One important caveat is that we don't currently have Canvas2D support, so if something uses that on the worker thread as a helper/companion, you are out of luck. I am currently prototyping this, as it is sometimes used by WPTs and the WebGL conformance suite to compare against the WebGL canvas.
With that said, there is a demo I found that should be working, at least on Linux, hopefully elsewhere too, as of the most recent nightly, that illustrates some benefit to this: