Awesome! Finally I've got an alternative to "Dictionary Extension" (https://addons.mozilla.org/en-US/firefox/addon/dictionary-extension/) of the pre-Quantum Firefox.
It maybe is a bit slow but that is okay.

I'd love it if the developer improves the UI a little bit (no fancy animation or background, etc. Just plain UI with the dialog easy to find with text easier to read). I think "Dictionary Extension" got it right with a plain and simple UI.