It's a viable replacement for the now legacy VimFx. Vixen is noticeably speedier and feels lighter.
Unfortunately it won't run on Firefox's own pages (like the Add-ons Manager) but as a WebExtension there's nothing to be done about this.
Currently it won't detect all the clickable elements VimFx detects, and there's no option to automatically disable it on certain sites (the last being a minor annoyance since it lets you disable it manually).
On the plus side it features console input while VimFx doesn't, and has a very simple configuration file that is easy to edit.

Overall this extension delivers what is expected and is the go to Vim addon for the new Firefox 57 and onwards.