Target Firefox and WebKit via hacky CSS.

moz-webkit.css

/*
  Read more here:
  https://developer.mozilla.org/en/CSS/@-moz-document

  For more browser-specific hacks:
  http://paulirish.com/2009/browser-specific-css-hacks
*/

@-moz-document url-prefix() {
  /* Put your Firefox specific code here. */
  span {
    color: black;
    font-weight: bold;
  }
}

@media (-webkit-min-device-pixel-ratio: 0) {
  /* Put your WebKit specific code here. */
  span {
    color: black;
    font-weight: bold;
  }
}