Why does the copy function in FireFox disappear from time to time?

Question

Browsers

Hello. For development, I use FireFox (the version does not matter, because the problem manifests itself in different versions), and sometimes, it simply loses the ability to copy something. From class name in firebug to website address. At least control + s, at least pkm-> copy. When you paste somewhere, the feeling that the buffer is empty, because. nothing is inserted and a nasty sound is heard.
Maybe someone knows how to overcome the attack?