How much RAM do you have free under About This Macintosh? Is Virtual Memory or RAM Doubler not an option for you? (And iBook RAM is dirt cheap. I stuck a 512MB SO-DIMM in my blueberryBook.)

FTR, I always test Classilla releases on my 1400, which has 60MB of RAM and is RAMDoubled to 120M, but I've tried to have some sort of sane minimum requirement (Mozilla is too memory-hungry to run in much less however).