How to solve the problem with cross-browser compatibility of the site?

Question

css

There is a site mittel.ru
In Chrome, everything is displayed correctly, in Safari with small bugs, in Firefox already with problems.
Please tell me what is the problem?

Answer 1

in main.js you hang on events not yet declared functions

window.onresize = bgresize;

... 

function bgresize(){
    ...
}

This is from what threw the Exception. FF 33.1.1

Answer 2

Go freelancing. There they will tell, and show, and correct ...

Answer 3

Tell me what is wrong for you? Or should we describe all the problems?