General

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. See issue #87.

Is it possible to customize the number of suggestions shown in address bar?

The number of suggestions is limited to the browser API, currently we can't customize it. Also, each browser have a different limit number.

Is it possible to modify the plugin keyword trigger? For example, can I trigger with r + tab instead of rs + tab?

Unfortunately, the browser's API doesn't allow the extension to change its keyword dynamically. You can customize your keyword by changing this line to build your own version.

Caveats

Why local file: rust 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 your-rust-doc-directory
$ python3 -m http.server --bind 127.0.0.1
Serving HTTP on 127.0.0.1 port 8000 (https://127.0.0.1:8000/) ...

Then set https://127.0.0.1:8000 as your local doc path.