CSS hack for Firefox

I need to place firefox specific CSS within one of my style sheets and can't remember the hack. .

This is the current CSS:

header {

    margin-top: -35em;

    min-width: 40%;

    background-color: #000;

    color: #FFF;

    font-family: Georgia, "Times New Roman", Times, serif;

    padding-bottom: .25em;

    opacity: .8;

    border-top-right-radius: 5px;

    border-bottom-right-radius: 5px;

}

I heed to change the padding-bottom: to a different value when Firefox is the browser, lets say it need to be padding-bottom: 0em

I can't remember how to do the hack, any help would be appreciated.

Thanks