HaroldTheBarrel

That's not really the full picture.

One of the lead Firefox extension API devs is also the primary Pentadactyl (and former Vimperator) dev. I'm sure they were aware of exactly what was needed for that and similar extensions. They also made it clear from their earliest announcements that they intended supporting these extensions.

It simply isn't a priority which may be fair enough.

Pentadactyl still works well on ESR.