See bug 1809843.
We have more than 600 tests that are skipped when running with Http/3 server.
We'd like to to investigate the reason and try to fix them.

Not sure about the best place to share this, but I know of two classes of issues:

• http/3 server does not register for all domains that the regular mochitest server does (bug 1827526)
• The tests run on https instead of http by default. If the test hardcode http (= same scheme as the ordinary http://mochi.test:8888 server), then the request is blocked on http/3 because that uses https://mochi.test:8888 by default. Example: https://bugzilla.mozilla.org/show_bug.cgi?id=1482983#c71
  • Here is the full list of files that hardcode http:-URLs: https://searchfox.org/mozilla-central/rev/85b4f7363292b272eb9b606e00de2c37a6be73f0/.eslintrc.js#624-2094