Targeting only Firefox with CSS

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.

1. Target all versions of Firefox

@-moz-document url-prefix() {
.content {
color:#000;
}
}

2. Target versions of Firefox 3

html>/**/body .content, x:-moz-any-link, x:default {
color:#000;
}

3. Target versions of Firefox 2

html>/**/body .content, x:-moz-any-link {
color:#000;
}