background images problem in firefox on photogra

My problem concerns the background image on firefox – sometimes it turns all pixelated and shows black and white lines – like a broken TV screen… (see screeshot attached) When I switch to another theme, everything works fine and in google chrome it looks ok too.

I don’t think it is due to incompatibilities with other plugins, cause I currently don’t have many installed and when I turn them off, the problem persists.

In the current version, I applied some additional code to the functions file to turn the menu into an accordeon. First I thought this could be the issue, but when I reupload the original theme with the regular menu I encounter the same issues !

You can view the site here: http://www.stenrudstrom.com/newsite/

It is currently in maintenance mode, if you need me to provide you with a subscriber’s or editor’s access, I could send it to you in a private message.

Thanks in advance for helping me out with this !