Platform

Any plans to support Safari?

Unfortunately, no. According to MDN's web extension compatibility chart: Safari doesn't support omnibox API, which is essential to this extension.

Caveats

Why local file: doc not work properly on Firefox?

For security reasons, in Firefox, file: URLs is an unprivileged URL, accessing to those unprivileged URLs are prohibited. See the MDN documentation for more detail.

Any workaround to support offline mode on Firefox?

Sure. A good choice is use http server! For example using python http.server module:

$ cd ~/Downloads/html-book-20200913/reference
$ python -m http.server
Serving HTTP on 0.0.0.0 port 8000 (http://0.0.0.0:8000/) ...

Then set http://0.0.0.0:8000/en as your local doc path.