Comment 17 for bug 343788

I suffer from the same problem - the only way I've found of returning things to normal is to run "sudo /etc/init.d/nfs-kernel-server restart" on my NFS server and then delete the FF lock files. Obviously this causes an interruption in the availability of NFS, and if you're using an NFS mounted home folder I've found this can lock up your X session on the client - so safest to log out of clients before the restart.