Seeing how the architecture of Firefox has undergone changes when switching to Quantum and WebExtensions, it is no longer possible to disable the built-in shortcuts using extensions such as the "Menu Wizard" or "Custom Shortcuts."
If you know how to compile Firefox from source, you can still do this by modifying the source. Download the source code, unzip it and edit:
path-to-ff-source-dir/devtools/startup/locales/en-US/key-shortcuts.properties
and change
inspector.commandkey=C
in
inspector.commandkey=VK_F1
If you are not familiar with how to compile Firefox from source, you can follow the instructions outlined here.
The source code for the latest version of Firefox can be found here:
https://archive.mozilla.org/pub/firefox/releases/ (do not skip / at the end, otherwise you will get 404 error).
Just select a release (e.g. 64.02) and click on the source:
https://archive.mozilla.org/pub/firefox/releases/64.0.2/source/
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