If you reboot the host and retry the steps above you will reproduce the FF download failure. If you close the VM and restart the VM, Firefox should be able to download the file. Immediately launch Firefox (38.2.1ESR or the latest 40.x release) and try to download a file (In our case, it is a epub file, ).
When we use Firefox from it freezes when we try to download a file. Guest OS is Centos 7.1 圆4 with the latest updates. Host OS is Windows7 SP1 圆4 with the latest updates. I have been running into what appears to be a bug with VirtualBox.