Version 5.1.7 is the last one compatible with Classic OS (System 7-OS 9.2.2). Version 5.2.3 is the last one compatible with OS X (up to Tiger 10.4.11, I think).
I'd use 5.x if I had to use IE, but I've been using Classilla exclusively on older Macs for the past few years since IE (any version) is a bit too old for decent web surfing.