We landed support for a form of CSS gradients on trunk a while ago, but we got considerable feedback that our syntax --- which was an incremental improvement of Webkit's syntax, which basically exposes a standard gradient API in the most direct possible way --- sucked. A bunch of people on www-style got talking and Tab Atkins produced a much better proposal. Since we haven't shipped our syntax anywhere yet, dropping it and implementing Tab's syntax instead was a no-brainer. So Zack Weinberg, David Baron and I did that (using a -moz prefix of course), and today it landed on trunk. It should land on the Firefox 3.6 branch shortly. It's unfortunate to land something new like this after the last beta, but in this case, it seems like the right thing to do instead of shipping CSS gradient syntax that we know nobody wants.
This does mean that anyone who's currently using -moz-linear-gradient or -moz-radial-gradient on pages is going to find that their syntax doesn't work anymore. Hopefully that's not too many people yet.