As diagnosed here: https://bugzilla.mozilla.org/show_bug.cgi?id=1515982 I am getting the white-browser behavior from Bug 1515826 when the RDD Process is enabled. The difference is that it never fixes itself for me: it stays hung for 5-10 minutes (after which I always stop waiting.)

It seems to be specific to my machine in some way, since dmajor did not experience the same thing.

This build has the RDD process starting at startup; and it hangs and has the white-browser behavior:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=a60ea79baf0f6fe2759eed38e53f65879ef0e74a

This build disables RDD and it works fine (including playing the av1 test movie at http://demo.bitmovin.com/public/firefox/av1/):
https://treeherder.mozilla.org/#/jobs?repo=try&revision=f4c12777c8354f290a7a21bf083f29d26ac8f021

This build makes RDD delay-start and it works fine until I try to play the av1 test movie after which the browser hangs:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=e352b1ed953460f5f9d0d62d610b6aa263d2cc99

I can't rule out that I have some weird debugging flag set in Windows that I forgot about; but no other builds (including a bunch of esr60 mingwclang builds I've been testing) cause behavior close to this.

If you leave the RDD process enabled at startup, but set MOZ_DISABLE_RDD_SANDBOX, does it work? (Trying to narrow this down to sandbox vs something else)

When disabling the sandbox, I get the same behavior for both starting RDD on startup and delaying starting it until it's needed.

What's on the RDD process's stack when it's hung?