From the Terminal

Using Meta instead of Ctrl for Cut, Copy, Paste shortcuts in Firefox for Linux

Set these in about:config

accessibility.typeaheadfind isn't blocked by Meta + C / Meta+V so we need to turn it off.

ui.key.generalAccessKey:   224
ui.key.accelKey: 91
accessibility.typeaheadfind: false
accessibility.typeaheadfind.autostart: false