Firefox mutilates page into two colums

i’ve recently begun work on an existing flash-based website and am having display problems with the new page. when the page was first uploaded, it viewed the same in ie and in firefox, altho it loaded much slower in the latter. now, with no provocation at all, the page displays in firefox as two separate colums, the header centered vertically on the left and the body swf on the right. can anyone tell me what went wrong and why?

are there guidelines available for testing a page to ensure it will view properly in all browsers?

also, any insight into or suggestions for improving the page load time will be greatly appreciated.

many thanks for your help and suggestions.