I use Firefox 40.0 on Ubuntu 15.04, I had some trouble with my history before so I created a new user. after that I faced with another problem: I can't see thumbnail images on new tab screen. after some search I found that's because Firefox cannot recognize caches and always I see "your web content cache is currently using 0 bytes of disk space". I think these 2 issues are related. P.S. creating another new user doesn't work neither and I'm not clearing cache data.