Keep in mind that if you do decide to use Firefox, you must use
Firefox 2.0.0.16, which will only be maintained through mid-December 2008. Firefox 3 was just released in June and is much better, but it requires Mac OS X 10.4.x or higher.
Upgrading your RAM to the maximum amount it can take would also be a very good idea, as Firefox eats up a lot of memory when running. There's no reason for you not to upgrade to 10.4.11 while you can still get it, unless you have specific applications that are completely incompatible or would cost a lot to upgrade to a compatible version. At some point you will have to upgrade, so if you wait until then, you might not like your choice, as it might require a new Mac to run that new version of OS X. When 10.4 is gone, you will have lost the chance to get it, so you gain nothing by waiting until then.
Mulder
Mulder