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.

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

As simple as it gets... and works perfectly. Lifesaver!

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