It lacks many features of Pentadactyl but covers the basics -- clicking links and text boxes with the keyboard. I have found it possible to replicate some of the things I missed from Pentadactyl through built-in features of Firefox I didn't know about (like the awesomebar filters I linked to above). Also, from using VimFx and accidentally hitting ":", i discovered the GCLI. It is a little clunky, but it is possible to define custom commands for the GCLI that have similar access to Firefox as XUL addons (and I think the GCLI is supposed to survive through WebExtensions).