divs shifted to the right in firefox only (moved WP to new server)

I have moved my site https://www.humanitariandesignbureau.com from MAMP (local) to a hosting server at online.net.
After the move, my div #inner_middle is shifted to the right by a good 300px with no apparent css cause (that i can find).

I am using a modified Starkers theme (I am not a programming pro, so the code is probably held together by paperclips and rubber bands in the eyes of more experienced WP users)

It is quite urgent(i know everybody says that), since (if you read the “news” post) we have a big event coming up and my site traffic is increasing by the minute.

Thanks!