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!