I have a mac (dual 2.7 GHz PPC G5, 5GB DDR SDRAM) running OS X 10.4.11. I run Firefox 3 on my other similar macs, but downloaded those some time ago. Website by default downloads Firefox 5.0.1.dmg which, after loading, (icon has the international symbol for "do not" across it)... pops up this dialogbox: "You cannot open the application "Firefox" because it is not support on this system". Firefox 4 won't even download AND doesn't indicate mac 10.4 as compatible.