After further investigation, the bug is in Gedit (or GTK?), not in Firefox: I have noticed that, when the bug occurs, I cannot paste (the data copied from gedit) in LibreOffice either. If I close gedit and open it again, I can copy / paste from Gedit to Firefox (or LibreOffice) again.
After further investigation, the bug is in Gedit (or GTK?), not in Firefox: I have noticed that, when the bug occurs, I cannot paste (the data copied from gedit) in LibreOffice either. If I close gedit and open it again, I can copy / paste from Gedit to Firefox (or LibreOffice) again.