No two browsers render alike (aside from Chrome and Safari as they have same engine). Issues like you're experiencing are template related (slowness is just due to IE being crappy) and should be adjusted with CSS specific to browsers. This is something web developers have been battling for years, cross platform support. Most browsers are standards compliant such as FF and Chrome while others not such much like ALL IE releases and Opera (latest Opera is pretty good now though). At any rate, you'll need to adjust the CSS of your template to accommodate other browsers and is not a bug.