Nouveau crashes Firefox and is described as not thread safe

Nouveau seems to crash Firefox reliably when enabling out of process WebGL.

It is described as not thread safe here: https://hg.mozilla.org/mozilla-central/rev/4ad85e996325

I could reproduce the problem: https://bugzilla.mozilla.org/show_bug.cgi?id=1671054

STR:

1. Download https://nightly.mozilla.org.
2. Open about:config (type it into the address bar), set gfx.webrender.all to true, webgl.out-of-process to true, webgl.out-of-process.force to true.
3. Restart Firefox.
4. Open https://webglsamples.org/aquarium/aquarium.html.
5. Main process crash: https://crash-stats.mozilla.org/report/index/e4d045c9-794a-4d20-b9d8-207120201013