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]
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).
cVim AFAIK implement the command line mode by injecting the command UI iframe and key binding events to every webpage as a content script. It works, but still less than ideal than what VimFX is trying to do by binding the shortcut globally.
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.
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]
[1]: https://github.com/akhodakivskiy/VimFx/issues/860