Today, I found when I copy from one application into my clipboard, the clipboard cannot be pasted into another application.
The next application has its own clipboard.
If I paste into the first application I see what I just copied.
If I paste into the second application I see what I copied perhaps an hour ago.
So, clipboard copy and paste has become application specific.
How do I turn this off?
EDIT, I'm on Mac 10.13.6, and I could not copy between Firefox and Chrome. Rebooting cured the problem.