I wish this would just let me add a bunch of clocks without having to pick timezones for specific places and then let me manually add/substract timezones with -/+ buttons or something. That would be much easier than that tree menu and having to pick the right timezones right away.
Also, let me pick another flag to my convenience.
The pop-out list from the button doesn't show times in the chosen time format. (Maybe the bar did, but that was before FF 57.)
Add option to hide the button to remove entries in the pop-out list.