Update browser - how/ which one?

I am currently using IE9 for a browser and many sites will not let me on because of an outdated browser, not supported. I have been told by young computer guys, (my son's friends, they live this stuff), that Firefox is a great browser and it is free. Please, can anyone tell me, is this true? Is Firefox a good browser, is it free, and how do I get it? Are there better browsers? Back in the day I was a great DOS man and a 386 was fantastic, I was there with the TRS80, but I am way out of my depth with todays technology. Someone please help, I really need it. Thank you

Sincerely,
old disturbed guy

Update browser - how/ which one?