When writing a Firefox web extension it's possible to use a default css for browser or page actions so that they are styled like other browser UI components. It's done by inserting:
Mozilla has a Style Guide which you can peruse to see how various elements and classes are used. The link to this Style Guide is in the
browser_style entry within the "Syntax" section of the
browser_action documentation page. A similar link is in the same location on the
page_action MDN documentation page. Personally, I would find it more appropriate for the information contained in the Style Guide to be hosted directly on MDN rather than on
If you are just interested in the elements and classes, you can find examples under the Components section.