Targeting only Firefox with CSS

Using conditional comments it is easy to target Internet Explorer with browser-specific CSS rules:

<!--[if IE 6]>
...include IE6-specific stylesheet here...
<![endif]-->

Sometimes it is the Gecko engine (Firefox) that misbehaves. What would be best way to target only Firefox with your CSS rules and not a single other browser? That is, not only should Internet Explorer ignore the Firefox-only rules, but also WebKit and Opera should.

Note: I'm looking for a 'clean' solution. Using a JavaScript browser sniffer to add a 'firefox' class to my HTML does not qualify as clean in my opinion. I would rather like to see something that depends on browser capabilities, much like conditional comments are only 'special' to IE…

OK, I've found it. This is probably the cleanest and easiest solution out there and does not rely on JavaScript being turned on.

@-moz-document url-prefix() {
  h1 {
    color: red;
  }
}

<h1>This should be red in FF</h1>

It's based on yet another Mozilla specific CSS extension. There's a whole list for these CSS extensions right here: Mozilla CSS Extensions.

Updated(from @Antoine comment)

You can use @supports

@supports (-moz-appearance:none) {
    h1 { color:red; } 
}

<h1>This should be red in FF</h1>

More on @supports here

Here is how to tackle three different browsers: IE, FF and Chrome

<style type='text/css'>
/*This will work for chrome */
#categoryBackNextButtons
{
    width:490px;
}
/*This will work for firefox*/
@-moz-document url-prefix() {
    #categoryBackNextButtons{
        width:486px;
    }
}
</style>
<!--[if IE]>
<style type='text/css'>
/*This will work for IE*/
#categoryBackNextButtons
{
    width:486px;
}
</style>
<![endif]-->

Here is some browser hacks for targeting only the Firefox browser,

Using selector hacks.

_:-moz-tree-row(hover), .selector {}

JavaScript Hacks

var isFF = !!window.sidebar;

var isFF = 'MozAppearance' in document.documentElement.style;

var isFF = !!navigator.userAgent.match(/firefox/i);

Media Query Hacks

This is gonna work on, Firefox 3.6 and Later

@media screen and (-moz-images-in-menus:0) {}

If you need more information,Please visit browserhacks