I get a 404 only in firefox/IE while Chrome/Safari works fine

I found a strange behaviour in firefox. While everything works fine with Chrome, firefox leads to 404 page on most links. So the main URL works, but subdirectory like url.com/news/ not. When I use private mode in firefox it works. When I clear the cache it works for like 1min then starts failing again. I checked Add-Ons in firefox (only skype click to call was active and I disabled it). Firefox version is 38.0.5

Same problem in IE11.

I installed http://ift.tt/pAgGCj as it is suggested in an other thread. But I can not see anything besides that when I want to open /de/news/ all I see is a request to /error/404.

Any idea what it could be or where to start to search?