Firefox 4 for PowerPC Mac
Apple no longer supports PowerPC Macs. So, its Safari (web browser) is stuck at version 4, while current version is 5. Similarly, the last version of Opera that support PowerPC Mac is 10.6x. Current version is 11. (See: History of the Opera web browser.) Google Chrome doesn't support PowerPC.
With your PowerPC being obsolete, can't you install Linux? Well, Ubuntu no longer officially support PowerPC neither. Unless you are prepared to spend days, and days afterwards to maitain it, the best choice is probably stay-put with your Mac OS X 10.4.x or 10.5.x, and hope to buy a new machine soon.