If you’re still using OS X 10.5.8 Leopard then you can still download Firefox for Mac 10.5.8. Mozilla Firefox is an excellent browser and replacement for Safari on your Mac plus it is completely free. To install it, simply drag it into your Applications folder.
Note – Firefox for Mac 10.5.8 may not be secure
From Firefox 17 until the latest version of Firefox, you need OS X 10.6 Snow Leopard or higher. However, if you’re still using a Mac running OS X 10.5.8 Leopard, you can still download Firefox for OS X 10.5.8 which is version 16 of Firefox. This is Firefox for Mac 10.5 to 10.5.8 and the last version that worked on Leopard but be warned that because it is old, it is not 100% secure as it is no longer updated by Mozilla anymore and many add-ons will not work with it either.