Based on the results from the Nightly experiment. We can enable ECH, ECH GREASE and ECH+QUIC in Nightly without fallbacks:

The Nimbus features are:

{
  "tlsEnabled": true,
  "h3Enabled": true,
  "forceWaitHttpsRR": true,
  "insecureFallback": false,
  "tlsGreaseProb": 100,
  "h3GreaseEnabled": true,
  "disableGreaseOnFallback": false,
  "greasePaddingSize": 100
}

Backed out for XPCShell failure on test_trr_https_fallback.js

Backout link: https://hg.mozilla.org/integration/autoland/rev/05faf93f2ab817b42119177d28b3ff8ca45f764e
Log link: https://treeherder.mozilla.org/logviewer?job_id=410510125&repo=autoland&lineNumber=4049

The pref was misnamed in a few tests and needed setting correctly in others.

Depends on D173642

Enable ECH and ECH-QUIC in Nightly without fallbacks. r=kershaw
https://hg.mozilla.org/integration/autoland/rev/2f1af2b7a36497db83400ff7047b78ff6e5a44c8
https://hg.mozilla.org/mozilla-central/rev/2f1af2b7a364

Fix misnamed pref in ECH tests r=kershaw,necko-reviewers,valentin
https://hg.mozilla.org/integration/autoland/rev/70dec260867a04a285867e0586434475917f480d
https://hg.mozilla.org/mozilla-central/rev/70dec260867a