fxcss inspect

fxcss inspect '#urlbar'
fxcss inspect '.tab-close-button' --dark

The same report, for a selector you already have. Useful for checking whether a selector still matches anything after a Firefox update — a common cause of themes quietly breaking.

If it matches nothing, it says so:

$ fxcss inspect '#urlbar-background'
no elements match '#urlbar-background' in this Firefox

That is a real example, not a contrived one: this repo's own example theme styled #urlbar-background by id, which many older themes still do. The id was replaced by a class, so the rule silently did nothing and the address bar rendered unstyled. One command found it; the fix was .urlbar-background.