Switched hosts and after upload the pages are rendering differently

Hi there, I’m really confused. I’ve just uploaded a site to a new host that I never used before for a client. The install has been a nightmare! Now the site is up for test and I have different margins in Firefox and different font sizes in IE. Is there any logical reason behind this? I can edit the CSS to correct but I would like to understand the cause and there’s nothing I could find online.