Changing the extension from PNG to JPG won't accomplish what you're trying to do -- that's like changing the extension from .DOC to .XLS to try and make a Word document turn into an Excel document.
A PNG and a JPG are not the same.
If you want to use the actual PNG image for the icon, then open the PNG image in Preview.app, select the entire image, select "copy", then do a "Get Info" on Firefox, highlight the icon, and paste.