How to disable Ctrl + Shift + C shortcut in Firefox?

Install Menu Wizard, click on Keyboard shortcuts, find key_inspector, delete the shortcut.

Install details here

Seeing how Firefox's architecture has seen an overhaul during the transition to Quantum and WebExtensions, it is no longer possible to disable built in shortcuts using an extension like "Menu Wizard" or "customizable shortcuts".

If you know how to compile firefox from source, you can still do it by modifying the source code. Download the source, extract it and edit:

path-to-ff-source-dir/devtools/startup/locales/en-US/key-shortcuts.properties

and change

inspector.commandkey=C

to

inspector.commandkey=VK_F1

If you are not familiar with how to build firefox from source, you can follow the instructions outlined here.

The source code for the latest firefox can be found here:

https://archive.mozilla.org/pub/firefox/releases/ (don't leave out the / at the end or it will give you a 404 error).

Just pick a release (64.02 for example) and click on source:

https://archive.mozilla.org/pub/firefox/releases/64.0.2/source/