I have never used this extension before, so I don't know that it ever worked. I do know it doesn't work for me right now, but perhaps it's user error (e.g. haven't performed a system upgrade in a few weeks [archlinux], did not relaunch firefox after installing [never have needed to]).
edit:
Apparently there is a working implementation downstream, which moves the source files so finding the fix through PR diffs isn't as easy as it could be.