The XUL extension I miss most is Pentadactyl. Tridactyl is a pale imitation, and still immature. Qutebrowser has potential, but still lacks critical extensions like uMatrix and uBlock Origin.
(To clarify: it does have an adblocker, but it's quite basic, only based on a blacklist of hosts. You can also selectively enable JavaScript for individual domains, but not yet based on both webpage and its origin like with uMatrix)