Inspecting the UI with devtools

Firefox's normal inspector only sees page content. The Browser Toolbox is the version that can inspect the browser's own UI, and it's off by default behind four prefs. fxcss turns them on in its throwaway profile, so in watch and pick you can just press:

• macOS — Cmd+Opt+Shift+I
• Windows / Linux — Ctrl+Alt+Shift+I

You get a full inspector over the browser chrome: hover to highlight, read computed styles, and live-edit rules to try things before committing them to your CSS. fxcss pick is the fast path for "what is this called"; the Browser Toolbox is the thorough one for "why is this rule not winning".