A strange cut-and-paste bug

I use cut-and-paste a lot. Often, I'm copying from a nedit text file, into the Firefox browser.

For a while now, that has stopped working. So I looked into it.

I can cut-and-paste from nedit into anything else, such as a terminal. That shows that copying from nedit to the clipboard is working

I can cut-and-paste from anything else into Firefox. That shows that copying from the clipboard to Firefox is working.

I don't understand it.