Why setting font-family for whole document breaks fa icons?

Question

I wanted to set the font-family for the whole document. I read that setting it to a body element excludes elements that don't inherit font styles (like form).

So i tried to use this:

*, *:before, *:after {
  box-sizing: inherit;
  margin: 0;
  padding: 0;
  font-family: 'Fira Sans', sans-serif;
}

But it breaks the font awesome icons:

icons not visible

But if set the font-family like this, it works:

* {
  font-family: 'Fira Sans', sans-serif;
}

Can you please tell me, what is the problem with the first code? Thank you!