Note that this only works if you’re using an Intel Mac – if you’re still using OS X 10.5 or 10.4 on a PowerPC Mac, then we recommend this article about how to install Firefox on PPC Macs and OS X 10.4.11. However, we strongly recommend you upgrade to at least OS X 10.6 Snow Leopard or higher as Firefox 16 is no longer updated by Mozilla and therefore not secure anymore. If you’re using OS X 10.5.8 Leopard then you can still download Firefox 16 for Macs which works on 10.5.8.
