Firefox 8 not launching on my PC Windows 7

I tried 5 times to download Firefox 8 as my PC says I'm using an old version and at risk of not being protected. I save and run and follow the prompts. It says complete and click to launch firefox 8 and when I do it takes me back to my original firefox that shows the warning that I'm using an out of date version. I keep doing this and same result. I shut the computer down and turned back on and still shows I'm using out of date Firefox. I checked in my Programs on Control Panel and Firefox shows that it's definetely downloaded. HELP I'm using Windows 7 on my PC and my antivirus is TREND