They both came out at the same time, I think Splash. I've been running 1.7.10 and the equivalent Firefox for about a week now. A new Opersa came out yeaterday, BTW (8.02).
I think it's a crying shame that, having invented the best browser on the planet, the Mox foundation plans to abandon it in favour of its retarded offspring. I know that the Seamonkey project exists, at least on paper, but I don't feel very confident about its ability to deliver the goods in an organised and timely manner. I might wind up eventually having to go back to Opera.