Retrieving margin-left in Firefox & Opera

Retrieving margin-left in Firefox & Opera

Stumbled onto this one: retrieving the margin doesn't seem to work in Firefox or Opera. 

This is code I'm using, which works fine in Chrome, Safari and IE:

  1.         var contentmarginl = parseInt($('#wrapper').css('marginLeft')); // fails
  2.         var contentwidth = $('#wrapper').width(); // works 
  3.         alert(contentmarginl);

The alert returns (in my case) 350 in the complying browsers and 0 in FF and Opera.

Is this a bug or my error? Is there a known workaround?

EDIT: Just tested in XP sp3 IE7 & IE8. Returns Not a Number (NaN)

Thanks.