If your iMac has a Core 2 Duo and 2 GB of RAM, you do have the option of updating to OS X 10.8.5 Mountain Lion and
running current versions of Firefox or Seamonkey. This is what I've done on my 2006 iMac 6,1 (recommend maxing-out the RAM to 3 GB if you haven't already). But right now I'm not aware of an up-to-date browser on Snow Leopard.