Before I reformatted when ever I opened a number of file extensions with firefox (.INI, .MP3, .WAV, and countless others) it would open with a program. But now it doesn't. I have to download MP3's, instead of Quicktime playing them (Which I have installed).
How would I go about remedying this?
You mean when you clicked them, it spawned an external program? Or they had browser plugins that played it in the browser?
Browser plugins. Quicktime popped up in the firefox window I opened it in and it played the MP3.
Go to FireFox's page, and click on "Plugins". It has instructions on how to install/configure them.