Re: WHY SHOULD I HAVE TO BUY A NEW COMPUTER TO RUN FIREFOX,WHEN I HAVE OS 10.5?
First off you should update to Firefox 16.0.2, which is the latest version of Firefox that will run on your PC. It isn't the most modern, but it's faster and more secure that 3.5.
We dropped support for Mac 10.5 just like we drop support for other old operating systems. The userbase on it is decreasing (due to users upgrading) and continuing to support it is hold back innovation for more modern Operating Systems.
As for why it is considered old, blame Apple. They released 10.5 in 2007 and stopped supporting it in 2009. Because you have an Intel Processor you can probably upgrade to Mac 10.6, which should be something like $20USD.