manifest.json provides ability to define keyboard shortcuts and it seems to be able to override shortcuts of Firefox itself. commands - Mozilla | MDN https://developer.mozilla.org/en-US/Add-ons/WebExtensions/manifest.json/commands However, actually it doesn't override Firefox's builtin shortcuts and some addons don't work as expected. For example: Disable Ctrl-Q and Cmd-Q :: Add-ons for Firefox https://addons.mozilla.org/firefox/addon/disable-ctrl-q-and-cmd-q/ Steps to reproduce: 1. Start Firefox on Linux (like Ubuntu). 2. Install the addon "Disable Ctrl-Q and Cmd-Q". 3. Hit Ctrl-Q. Actual result: Firefox quits itself. Expected result: Firefox stays there.

This is another testcase. When you load the manifest.json from about:debugging, Ctrl-N(Cmd-N) should be disabled but actually it still work. This is tested on Windows and Ubuntu. On the other hand, Google Chrome also has same "problem". Shortcuts same to a builtin one with no action doen't deactivate Chrome's builtin action.