Give float:left to "#main_menu a.l1".
The other problem is that IE6 doesn't understand "a.active.l1" or "a.trail.l1" properly. In This case I suggest you use "li.l1 a.active" and "li.l1 a.trail" instead. (There may be more places that have the problem, but that's the rule that's visibly symptomatic.)
P.S. I am writing a reply to your email.
