Recenlty we ran into a glitch using an online editor in a website CMS
with the Firefox browser. Every time we tried to paste plain text into
the editor, the editor would crash or display an error message. Other
browsers worked fine (chromium, Seamonkey), and Firefox worked fine on
other workstations. It turns out it was the method some editors use to
process clipboard events. In 'about:config' the offending browser had

dom.event.clipboardevents.enabled

set to 'false'. Resetting to 'true' fixed the issue (the default setting).
I had probably set this, or some no longer in use add-on/extension had
set it, to enforce additional security or privacy.