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