I have no issues with the process. see screenshot Did you try with Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf

MODERATOR NOTE:

Recommendations to flip dom.events.testing.asyncClipboard to true are a fundamentally bad idea. This pref was meant for automated testing, and bypasses all security/permission checks on clipboard access.

If you toggle this pref, any site has full access to your clipboard all the time without asking you or you even noticing so we suggest that you do not touch that pref.

Had the same problem with firefox and outlook web.

My solution was:

   open "about:config" in firefox (type it in the webadress line and hit enter)

   accept the risk in the pop up

   type "clipboard" in the search line

   change the following settings (as shown in the image attachment) from false to true (klick on false)

   restart firefox (close it & open it again)