the best extension for overwriting websites that lock their clipboard.
Just install it, restart your Mozilla, Ctrl-C whatever you want to copy, place your cursor on the form you want to overwrite, and voila, a paste popup will appear, and they will overwrite it.

The perfect addon. It just works.

This extension should be the #1 result since it does not need permission to read all websites.

Once it saved me from losing job application opportunity where paste option were disabled.