Hallo,
vielleicht wäre der "iCab" einen Versuch wert?
http://www.icab.de/index.html

Habe zwar selbst keine Erfahrung damit, aber wenn schon die Mozilla-Entwickler den Umstieg auf iCab empfehlen:
"Due to the lack of developer interest, build machines, compilers and testing resources, the last mozilla.org software built for Mac OS 9 was Mozilla 1.2.1, released in December 2002. However, versions of our software that old are not recommended for security reasons. Therefore, for Mac OS 9 users, mozilla.org recommends iCab - which is, as far as we know, the only graphical browser currently maintained for Mac OS 9."
Quelle: http://www.mozilla.org/download.html#os9

Grüße,
Klaas