Can't find how to get Firefox for iphone 3GS?

Firefox isn't available for iPhone (in fact, Apple doesn't allow alternate browsers on iPhone at all.)

Thats not true. There are several alternate browsers available on the appstore. My favorite is Mercury.

As TEW said there are several other browsers avaliable and even one made by the well known company Opera called Opera Mini. Also Mozilla have said: "We would make Firefox for iPhone if we could. The ball is in Steve’s court!" If you want to see where I found that info then visit: Mozilla

The browsers available for iPhone either use the built-in WebKit rendering engine, or do rendering on a server like Opera Mini.

Firefox includes its own rendering engine and JavaScript interpreter, which is forbidden by section 3 of the iPhone SDK agreement. Under the current agreement, we could not distribute Firefox for iPhone without replacing essentially all of its code.

You should distribute it on Cydia instead of the Appstore.