How to remove Firefox's dotted outline on BUTTONS as well as links?

I can make Firefox not display the ugly dotted focus outlines on links with this:

a:focus { 
    outline: none; 
}

But how can I do this for <button> tags as well? When I do this:

button:focus { 
    outline: none; 
}

the buttons still have the dotted focus outline when I click on them.

(and yes, I know this is a usability issue, but I would like to provide my own focus hints which are appropriate to the design instead of ugly grey dots)

Solution 1:

button::-moz-focus-inner {
  border: 0;
}

Solution 2:

No need to define a selector.

:focus {outline:none;}
::-moz-focus-inner {border:0;}

However, this violates accessibility best practices from the W3C. The outline is there to help those navigating with keyboards.

https://www.w3.org/TR/WCAG20-TECHS/F78.html#F78-examples