https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly

I could not find Ctrl+Shift+M on the above page. What does it do?

That is not a recent change as "Ctrl+Shift+M" has been the shortcut for responsive design mode for years.

You can disable all developer tools in Firefox if you have no need to use them by setting "devtools.policy.disabled = true" on the about:config page. You need to close and restart Firefox to apply this change.

You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue".

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Hello, recently Ctrl+Shift+M has been grabbed by Firefox but I had already mapped this shortcut to a command in my desktop, how do I disable this shortcut in Firefox? Thanks!

Shouldn't the system-level application grab the shortcut before it is sent to Firefox? Maybe something is not working correctly with that application.

I think the way Windows works is the 'on top' program gets it first. Then it goes to the system if the program does not take it.

Hello,

Ctrl+Shift+M shows the page in mobile phone mode (I guess, since the page is much reduced in size).

I hadn't thought about the fact that my shortcut has in fact been removed from my setup (but I don't know why!!) so it's not a problem with Firefox itself but with my desktop. Thanks for having pointed me this way!