DevTools Auto Open
Add-on for Firefox browser.
Automatically opens Firefox Developer Tools when new tab / new window is opened.
If user closed Developer Tools on tab then add-on won't reopen Developer Tools on this tab.
By default Developer Tools will be opened on tab ready event: when the DOM for a tab's content is ready.
If you need then Developer Tools may be opened earlier, immediately when a new tab is opened, for that open
about:addons > DevTools Auto Open > Preferences and check option On tab open.
On tab open is disabled by default because produces bugs
#1,
#2.