Try harder to adapt to the browser chrome appearance
According to https://developer.mozilla.org/en-US/Add-ons/WebExtensions/manifest.json/browser_action#Syntax there is a browser_style attribute that can help to get a better match with the surrounding appearance. This is important for the font size at least, because users can have set a global interface font in the OS that is bigger than normal. On Linux at least, you can also choose a specific font for the UI of apps and most toolkits obey that nowadays. AMR looks a bit strange in that setting.
Currently this key has no effect when not in Firefox, so good fall-backs would have to be implemented (or maybe some polyfill exists).