Firefox and mobile css problems

I have just started a new website for my organization, www.toppenkurset.no. Most browsers views the site OK, especially Safari and Chrome. But in Firefox everything is out of order. I have done some small changes in the css, but nothing big, which should be the source of the problems. In addition, the website is not so good on mobile phones either.

Do anyone have good css skills, and have time to give me a hand?
If you need any css files, just tell me, and I can direct you to them.