hello,
i`m the developer from the lcd-clock, is it possible to rewritten in webextensions or does the new API not support directly a clock in the Toolbar?
https://addons.mozilla.org/de/firefox/addon/lcd-clock/
AFAIK, at the moment, it is not possible to have that on the tool bar. You can have a Toolbar icon with a badge that can only contain 4 characters (not enough for a clock)
You can recreate the icon with numbers (like 2 numbers as icon and 2 numbers in the badge) or you can have a pop-up with the clock.