simple css browser difference

I say simple since I am sure this has been covered a million times.

I created a page with CSS testing it through IE6. Layer positioning came out
perfectly aligned.

THEN....

I tested it in Mozilla 1.7.1 and things didn't line up just right. I copied
the page and renamed it mpage (for mozilla) - copied the CSS file and made
the changes in positioning to line things up correctly.

QUESTION:

How do I incorporate both css files for users of both browser to see the
page correctly? There has to be something in JavaScript that identifies the
browser and then points to the right css file - RIGHT??!!

Kitty