How can I disable Ctrl+Q for all applications (system wide)?

The best way is to disable the keyboard setting in your specific application. For Firefox there is an extension keyconfig. This allows you to change keyboard settings.

If you want to disable Ctrl+Q for your whole system, I would suggest to make a new setting:

1. Go to System -> Preferences -> Keyboard settings
2. Click Add
3. Give it a name like fake setting and enter /bin/false as command. Apply your changes.
4. Click on 'Disabled' and press Ctrl+Q.

Now you should have a new entry with your applied name and your keyboard setting. Every time you press Ctrl+Q your system will run the command /bin/false which basically does nothing. So this is a workaround to disable the setting.

Probably inserting the shortcut under System -> Preferences -> Keyboard Shortcuts and "disabling" it should do the trick.

You can go to System->Preferences->Keyboard Shortcuts and assign this combo to an unused action. For example I assigned CTRL+Q to switch to workspace 12 (No, I don't have 12 workspaces).

As a result this system wide setting overrides application one and CTRL+Q no longer closes firefox (likely the combo never reach Firefox). It just do nothing.