Firefox doesn't have an equivalent to IE's non-standard use of conditional comments (as far as I know).
On the server side, you can check your user agent with something like this;
It's not a guarantee of course; user agent strings are easy to forge.
$isFirefox = preg_match('/firefox/i',$_SERVER['HTTP_USER_AGENT']) ? TRUE : FALSE;
echo "<p>Yep, using Firefox</p>";
echo "<p>Nope, some other browser</p>";
You should have a really good reason for discriminating by browser.