Places a border around the browser chrome when the page loads with the colour, thickness and border style you have chosen, allowing you to recognise what environment you are in easily.
A simple string input into the preferences is matched against the URL.
The border is placed in the chrome so does not affect the document's rendering in any way.
Although this is in the Web Development category it is simple enough to be used by any Firefox user who may need to distinguish between different environments visually.
It has been inspired by terminal windows with different colours for different environments!