Audio Desk 2.04 and Mach Five 1.2.2 or higher are required for Tiger compatibility.
Firefox 3.6.20 is available (and current) for PPC Macs.
When the Mozilla people stop maintaining Firefox for PPC, you can migrate to Ten Four Fox.
There are very few (if any) pieces of Power PC malware floating about.
Any attack is likely to involve Intel code, which wouldn't run on a G4 box.
In particular for Ten Four Fox -- I'm setting up a 15" i Mac G4 as a garage Mac and was hoping for a modern-ish web browser. (As soon as I chase down some RAM for it I'll be able to upgrade it to 10.5.x.