For Firefox:
You can use the --devtools command line --devtools ( introduced in Firefox 45 ) to launch Firefox with the developer panel open. This works with or without a URL to open.
Note that this does not work when the call 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, you need to start Firefox with the --no-remote flag. In this case, you will also need -P to specify an alternative profile to start with, since the default profile is usually used.
However --devtools applies only to the window / tab that is opened at startup. It seems that there is no way to automatically open the developer tools in new windows / tabs opened later, as in Chrome.
source share