The preference javascript.options.showInConsole controls whether errors or warnings in chrome code are shown in the Error Console.

Values

false
Only errors and warnings from content code are shown.
true (default)
Errors and warnings from both chrome and content are shown in the Error Console.

Note: Since the Web Console was introduced in Firefox 4 specifically for debugging content, the default value for this preference has changed to true as of Gecko 2.0.

See also

Setting up extension development environment