fetchLater support in Firefox

[nicjansma]

Tracking issue for fetchLater() support in Firefox.

[tunetheweb]

Bugzilla bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1936180

[nicjansma]

Position: Positive

mozilla/standards-positions#703

[mangs]

WHATWG fetchLater PR: whatwg/fetch#1647