you can check the following snippet, the issue is that Roboto will not show properly on firefox on greek characters.
body {
font-family: Roboto, sans-serif;
}
<link href="http://fonts.googleapis.com/css?family=Roboto" rel="stylesheet" type="text/css">
<h1>This is a test</h1>
<h1>Αυτό είναι ένα τεστ</h1>
It would appear that you need to add the correct subsets to the font API call. Try using this instead
http://fonts.googleapis.com/css?family=Roboto&subset=latin,greek-ext,greek
This appeared to work for me in firefox.