You can’t style options, as they’re browser-dependent and there isn’t an agreed standard. You’d have to use a replacement, which provides the functionality with other components.
Then, to keep it open, you should be able to toggle classes and states in the inspector. I’m Firefox, it’s above the style inspector, labelled as .cls and :hov, I believe (I’m on mobile at the moment).
Seems like it. The article also mentioned that there are only 73 pairs available, so it sounds like Mozilla has to explicitly define what Chrome extensions corresponds to what Firefox extension.
Instead of importing the Chrome extension directly, Firefox is installing the Firefox version of the extension from Mozilla’s own extension store.
Seems like it’s just for making the switch from Chrome smoother, rather than being useful for long-time Firefox users.
You can a bit, but that will be limited and will differ by browser. Look at something like this to do it: https://medium.com/@ross.angus/styleable-select-box-replacement-in-pure-css-and-semantic-html-1f38a400b285