<style type="text/css">
@-moz-document url-prefix(){
    H5 { color:red; }
    P { margin-left:20px; }
    /* other special styles for FireFox here */
}
</style>

http://www.apphp.com/index.php?snippet=css-targeting-firefox-only

This solution is based on Mozilla specific CSS extension. Other browsers will ignore this rule.

Targeting Firefox With CSS

css

CSS