The best possible candidate is TenFourFox, especially if you can get the current beta (it's around Firefox 38 iirc).
These are the only two browsers that are actively being developed under OS X, but the other one (Leopard Webkit) is 10.5 only. So your only chance is TFF.
You could try Classilla, BUT Classilla is still playing catch up with all of the Mozilla updates. Plus, it works on the innards of OS 9 instead of X (meaning you need Classic running). It's going to be awesome when Classilla is caught up and right now it's a pretty usable state, but I don't think it'll work for you.
TenFourFox is by far the most updated and feature-full PowerPC browser. If you need a feature, the best place to look is TenFourFox. Cameron Kaiser is doing a great job with keeping it up to date for PPC users!
Keep in mind that Roccat links against the default WebKit frameworks in Leopard unless you update them to Leopard WebKit. If you don't, you are looking at compatibility and security issues.