For starters, I recommend fixing the
markup errors reported by our online validator. Some of them are serious structural problems.
It looks like you haven't left enough room inside div#content for everything (div#shout, div#recent, div#forumbt, and div#oekakibt). You seem to be attempting a pixel-precise layout, but you haven't specified all the margin & padding values. Different browsers have different default margins/padding, so you get different results.