Note: If you want to debug a specific add-on that is restartless or SDK-based then try the Add-on Debugger. Use the Browser Toolbox for add-ons that are neither.
The Browser Toolbox is not enabled by default. To enable it you need to check the settings "Enable chrome and addon debugging" and "Enable remote debugging".
Open the Browser Toolbox through the menu button and the menu items "Developer" then "Browser Toolbox".
Click the menu item and you'll be presented with a dialog like this (this can be removed by setting the
devtools.debugger.prompt-connection property to false):
Click OK, and the Browser Toolbox will open in its own window:
From Firefox 37 onwards, you can debug chrome: and about: pages using the normal Debugger, just as if they were ordinary content pages.