xIFr v2.8.0
Some code refinements, refactoring and preparation for future MV3 migration.
Plus an attempt to fix disappearing menu-item caused by https://bugzilla.mozilla.org/show_bug.cgi?id=1817287 (A work-around by trying to re-define menu-item at every browser startup).
Possibly (also?) fixing menu-item that has disappeared by other reasons.