Most upto date Browser for Mac Os x 10.5.8
Firefox 3.6. 28 is the last version of Firefox that works with Mac OS X 10.4 (Tiger) or with Mac OS X 10.5 (Leopard) on a computer with a PowerPC processor. Firefox 16 is the latest version that supports Leopard on Intel-based Macs as Firefox 17 requires 10.6 or newer.