On the Classilla wagon: Up until a few days ago my only experience with Classilla was on a 180mhz powerbook 2400c. Wasn't incredibly impressed but it functioned. Then, I used it on an 867mhz Quicksilver G4 and WOW!!! The performance was snappy and everything ran and rendered exactly as I expected within reasonable time. Even that Google animation on their main search page was fluid. I was just looking last night on their website to see if they took PayPal donations. Definitely worth $15.
Back to the iMac G3, very nice find! When OS 9 was still my daily driver OS (up until 2005 on my PowerBook) I used a version of Netscape 7. I want to say that 7.0.2 was the version I was using. At the time Mozilla had a portal full of legacy downloads, that's where I found it. Honestly though, going to have to second that vote for Classilla. Since that is a slot-loading model, I'm going to take a bet that it is running in excess of 500mhz. If that is the case I'd say there is a good chance of acceptable performance.