Firefox / Chrome Debugger - Open Automatically

Chrome added the option to "Auto-open DevTools for popups" in early 2016. I'm seeing the option in Chrome 51.

Open DevTools Hit F1 to open settings Scroll all the way to the bottom, and the option is under "DevTools"

For Firefox:

You can use the commandline flag --devtools (introduced in Firefox 45) to start Firefox with the developer toolbar open. This works with or without specifying an URL to open.

Note, however, that it does not work when the invocation opens a new tab in an already running instance of Firefox (the default behavior if Firefox is already running). If you already have an instance running, you will have to launch Firefox with flag --no-remote. In that case you will usually also need -P to specify the alternative profile to start with, because the default profile will typically be in use.

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However, --devtools only applies to the window/tab opened upon startup. There does not appear to be an option to automatically open the developer tools in new windows/tabs opened later, as in Chrome.