Bug 571367 switched Firefox universal builds for Mac to be Intel 32/64bit instead of PPC/i386. SeaMonkey should do the same, but currently lacks the machines, as it looks like the new ones are only buildable on 10.6 machines, at least with the configurations we usually have on Mozilla machines.
We still should investigate what we need to be done for that and get it going, but it might take some time.
For the moment, bug 599796 work should ensure that PPC/i386 universal builds at least continue to build.
I've been working on this in bug 558168 work, we should have them up and running mostly now, but I still need to care about the update server.
This has been done now, we have switched over the builds on trunk. I hope they work well.