I have looked at bug 1532323 already. That bug is only websockets over http2 bug and it is not a http proxy bug.
Bug 1543502 does not really blocks this. I am sure that that bug is invalid, because TRR has a systemPrincipal (It must have) and webExtension do not get any onBeforeRequest, On... for requests triggered by a SystemPrincipal.
This project has not publicly announced yet. Let's flag it as 'mozilla-confidential' for now.