I have that issue occasionally, but repeating the action once usually helps, except on rare occasions where i may try it up to 3 times. The behavior in my case is not limited to firefox. I hate repeating this for many answers, but if you feel like it is an issue specific to ff or it is new in ff for you, try sync or backup, then uninstall ff and reinstall. Until you get a resolution, you could probably go into apps settings and force stop ff rather than rebooting your phone. I am sure it would be significantly faster and accomplish the same thing. Lastly, to what version of Android has your S4 updated? It is wholly possible that newer versions of ff aren't tested as well against some older Android versions.