I played with the copy&paste demo you posted (http://kevingrandon.github.io/Firefox-OS-Clipboard/) and noticed that you can actually cut (i.e. remove) text from read-only parts of the web page.
Reproduce: Long click on the "Welcome" word in your very first sentence of the demo page. Click on cut.
P.S.: I like your work, very nice :)
I can confirm it.