I need and like this extension for being a no-permissions, simple solution. Sadly, it is broken and does not display all digits of 24 hour time. Also, it is too small to be easily readable and to align with my toolbar.
If this ever sees further development or someone else wants to make something similar: It would be nice to have default background colors matching Firefox's default themes.

For some reason, the numbers aren't aligned correctly and the last digit is usually partially cut off.