bug(button): show dotted border on firefox

[jefersonestevo]

Bug, feature request, or proposal:

bug

What is the expected behavior?

The button should behave consistently in any browser

What is the current behavior?

In firefox, the button is showing a dotted border when clicked

What are the steps to reproduce?

Plunker
** Open in firefox

Image:

Which versions of Angular, Material, OS, browsers are affected?

Firefox 52.0

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