ESR140: Revisit Extensible Priotization Scheme for HTTP
Followup to #42600 (closed), whose mozbug was backed out from ESR128 because of webcompat (see https://bugzilla.mozilla.org/show_bug.cgi?id=1915134).
Revisit in esr-140. Metabug: https://bugzilla.mozilla.org/show_bug.cgi?id=1865370