I have the PowerMac G4, OS X 10.4, and I don't seem to be able to upgrade anything anymore. I downloaded a new Firefox, but can't use it. Tried to upgrade to 10.5 but can't. Before I give up and buy a new MacBookPro, does anyone have any tips?

Does your Mac meet the requirements for Leopard?

http://www.tuaw.com/2007/10/16/leopard-system-requirements/

Ciao.

Firefox no longer supports PPC Macs or odler versions of OS X. Instead use Ten4Fox:

You can download TenFourFoxG5 from here:

http://www.floodgap.com/software/tenfourfox/

You should be able to install leopard anyway if you have enough memory.

google:

imac 800 leopard install

"You will probably be able to run Mac OS X 10.5 on an unsupported machine with XPostFacto, but it may be too slow to be usable, and it will not be available on the Mac OS X release date."

https://discussions.apple.com/thread/1181038?start=0&tstart=0

change clock speed for install. see this post:

Mac Man 82

http://forums.macrumors.com/showthread.php?t=392106