Type "About:config" into the address bar near the top of the Firefox window, and press "Enter."
Click the "I'll Be Careful, I Promise" button on the warning page that appears.
Type "Errorconsole" into the filter box at the top of the page.
Double-click the "devtools.errorconsole.enabled" preference to enable it.
Close all open Firefox windows, and re-open Firefox.
Click the "Firefox" menu button, hover the mouse cursor over "Web Developer" in the menu, and click "Error Console."
Click the "Errors" button at the top of the error console window to show only errors, not warnings or messages.
Navigate to the Web page with the errors, and read them from the error console window.
- check Click the "Tools" menu if you don't see the "Firefox" menu button.
- close The About:config page contains a variety of options users shouldn't modify; be careful when using it.