You can see the implementation here:
In FireFox on Mac, that circle is round/solid. I want it either dashed or dotted.
What makes it even weirder is that the same property (i.e.
border: 4px dashed #000) works fine on an HR. So why does it not work when border-radius is involved?
This is so bizarre to me.
It's a bug, WebKit had a similar issue but it was fixed in June. Here are all the other outstanding border-radius defects in Firefox.
This recipe can be found in it's original form on Stack Over Flow.