I have been using Fedora 20 KDE with NFS mount on /home for months now without any problems when running firefox. One day I decided to try NFS cache (cachefilesd) to improve the performance. Immediately after enabling it, on the next boot when I tried to run Firefox the process hanged with Disk Sleep. Of course the process cannot be terminated and the only resort is a system reboot. The thing is that firefox runs without problems (disk sleep) around 50% of the times and 50% it hangs at initialization. Also if Firefox manages to initialize properly it then does not hang while operating. So it only happens at startup of firefox and around half the times I run it. Did not find anything odd in the /var/log/messages. Please note that all other problems (and other browsers) run every time properly with NFS cache, so this behaviour is unique to Firefox...