Saka Key is okay, but I am rating it 3 out of 5 admittedly not on its own merit (which I hope developers may forgive me for) but because compared to VimFX, my preferred extension before Firefox 57 arrived, Saka Key lacks in certain areas:

1. The escape combination (to be able to transparently forward all keystrokes to the web page) has to be re-typed every time you switch focus back to the tab where you need to escape keystrokes again. This is very inconvenient, and combined with the next lack-of-feature on the list, makes for a truly irritating experience for pages where you need to disable Saka Key temporarily through escaping.

2. There is no way to blacklist certain URLs -- Saka Key is either enabled for your entire browser or not at all. Since keystroke escaping described above does not persist between even switching tabs, this really hurts.

Other than that, Saka Key does just what it says it does. But since it is a fresh start with Web Extensions API, I understand it's not there yet. It's not authors fault they have VimFX to compare to :) Best of luck, Saka Key!