Target specific browsers CSS

Hi – I have an intro paragraph with a drop cap. The CSS selector ::first-letter works fine in all browsers except Firefox, where the styling is a little off.

So, I try adding this @rule to the Selector, in the Code section, however, a warning shows saying this rule is unknown. If I click into another selector and back into the ::first-letter selector, that @rule then disappears.

@-moz-document url-prefix() {
article:nth-of-type(1) p:nth-of-type(1)::first-letter {
margin-top: 7px;

}
}

If I add this @rule to another external style sheet, it works.

Thanks. Gavin.