i guess there are some issues with the x-corporate theme and the firefox browser. who else has some disyplayproblems with browsers?
It would be very helpful if you could explain your displayproblems in detail.
Screenshots please 
this is how the theme looks in firefox: Bildschirmfoto vom 2017-04-04 20-56-34
Bildschirmfoto vom 2017-04-04 20-56-48
in chromium it looks like in the demo, but only in cromium. vivaldi has the same issues like firefox
looks like the surrounding list element is not clearing the previous one. Either each element should be display: block; or if each item is floated, then they need to be ‘cleared’ https://www.w3schools.com/css/css_float.asp
You should report this on the theme repo issues: https://github.com/tranduyhung/grav-theme-x-corporation/issues