VimFX works very well in FF51.
Some of us who use Vimperator use it not only because of the key binding, but also for the command-line, ability to modify the UI (e.g. :set gui=none), ability to put .vimperatorrc in .dotfiles and use it across multiple systems, and such. As far as I know, VimFX still not capable of all these functionalities (some due to difference in mission, e.g. not modifying the UI.)
That said, WebExtensions at its current state still couldn't support all of VimFX's current feature either.[1]
https://chrome.google.com/webstore/detail/cvim/ihlenndgcmojh...
I also use this in Chromium -
https://chrome.google.com/webstore/detail/wasavi/dgogifpkoil...
Note that they do very different things from each other AND that they are running Chromium which is a SUBSET of the plugin features that will be available in Firefox (if all goes well).
There's also an ongoing discussion to try to port Vimperator to WebExtensions[1], which having everything as content script might be one way to do. However, it still doesn't give an ability to modify the UI as XUL/XPCOM Vimperator does.
[1]: https://github.com/vimperator/vimperator-labs/issues/705
However there is one killer feature for me in Firefox, that is the fact that I can:
nmap <C-j> 10j
nmap <C-k> 10k
Chrome has these keys already bound and there is no way to unbind them.
I am adamant about using this configuration because it matches my .vimrc - so I use the exact same navigation in my browser and my (arguably) IDE.