CSS and JavaScript question?

I have a website that looks different between IE and Firefox. Right now, it looks fine in IE, but is off in Firefox. If I fix it to look right in Firefox, it looks off in IE. I know there is probably some way I can pinpoint the problem in my stylesheet so that both are the same, but if I can't do that, can (and should I) use JavaScript to detect the browser, and just have it use a different stylesheet depending on which browser the user has? Thanks!