if you comment out this line:
/* url-prefix("file:///"), */
be aware that your local html file won't be rendered as they should be
EDIT: nov 2008, I actually don't use it anymore, I found a way better way to achieve this, without this downside mentionned above:)
see the about:config tips on this page: http://userstyles.org/styles/9631
.ad {
/* Your ad can be here instead. 10K unique visitors for $5/day. Mail me. */
}