Ctrl-Shift-C-Should-Copy
Potential Firefox extension to intercept Ctrl+Shift+C, block opening developer tools, and copy the selection to the clipboard.
Available on AMO here:
https://addons.mozilla.org/firefox/addon/ctrl-shift-c-should-copy/
To view the contents of the add-on as it would be downloaded from AMO, see: https://robwu.nl/crxviewer/?crx=https%3A%2F%2Faddons.mozilla.org%2Fen-US%2Ffirefox%2Faddon%2Fctrl-shift-c-should-copy%2F