No outline on focused inputs in Firefox

问题:

It seems Firefox no longer displays an outline/highlight on focused elements. When I searched this topic, I got the impression that Firefox did this in the past by default, but discontinued it. Chrome, on the other hand, outlines the selected input with a light blue border:

I'm trying to create a Greasemonkey script in Firefox that allows users to scroll through checkboxes/radio buttons using the arrow keys. Whichever input is focused should be highlighted so users know it is selected.

Is there anyway to do this in the browser itself, i.e. make it so Firefox highlights the selected elements on pages by default? (Other than downloading a previous version of Firefox or adding CSS to my web site (I need this to work across sites))

I'm using Firefox 40.0.1 and Chrome 55.0.2883.87