Firefox, SeaMonkey, Pale Moon AND GreaseMonkey ONLY! Disable (remove) copy text protection and allow copy for all sites.
WARNING: MAY BREAK PAGE or ADDONS FUNCTIONS. (especially online text/graphic/scheme/etc editors).

NOTE:
1. IF site have operations: 'drag', 'dragend', 'dragstart'
when you drag text, use hot keys "alt + selection" for text or add operations to array in source code.
2. IF site block context menu you may use hot keys "shift + mouse right click" on element.

Links:
GreaseMonkey https://addons.mozilla.org/en-us/firefox/addon/greasemonkey/