I've tried your code and this is my sample code.
<link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Fira+Sans:300,300i,400,500,600,700,800&subset=latin,latin-ext">
<style>
h1 {
font-family: 'Fira Sans', sans-serif;
}
</style>
<h1>Większość szkoleń prowadzę</h1>
Fiddle:http://jsfiddle.net/fbuzrmL5/
You could see it works well.

Could you show your issue with a screenshot or describe in detail?
Or you could refer to this link:How to make this Google Font work on Internet Explorer 11
From google fonts: "Note: For best display in IE, make the stylesheet tag the first element in the HTML section. In IE, if the link is placed after tags, the entire page will block and not display anything until the font is loaded."