This is the one that i search for.

For the users who cannot read "military" time, learn to adapt kids, the world does not revolve around you.

无法使用。但估计是浏览器的问题。

I forked this and made it 12 hour format

https://addons.mozilla.org/en-US/firefox/addon/clock-hours-12/

Exactly what I was looking for. Could do with customization, maybe font choices, but those are extras.
This is a clock that's simple and works. Even the hour/minute parts are separated, presumably to fit the extension icon size, but also allowing you to have a less precise "hours only" clock depending on how you prefer to manage your time.

If you're like me, and using this with a tab sidebar extension in full-screen, you can actually customize the Firefox toolbar (including the forward/back/refresh buttons), and have it sit above your tab sidebar! Simplicity wins!

Can be updated switch mode(drak light) custom text color