Internet Explorer and Firefox will not conflict each other.

You can have multiple internet browsers installed on your computer without any issues.

When you install Firefox (you can get the version for your operating system and language here), watch out for the option to make Firefox your default browser. If you want links in external programs to start in IE, keep IE as your default browser and start Firefox when you want to use it.

If Firefox takes over as default browser and you want to switch it back to IE, check out this article from Microsoft: http://windows.microsoft.com/en-us/internet-explorer/make-ie-default-browser

P.S. I can't understand why Yahoo wouldn't support IE9. I think that's still a popular version of IE.