Works and, unlike AT Price Tracker, is available to Firefox users. But two things:

1. Feels like I'm the one finding all the price rises. There are many more AT Price Tracker users and its price history is correspondingly longer and more accurate. This is hardly Motorwatch's fault but it's an obvious difference

2. On Firefox, installing the extension mangles the AutoTrader CSS such that many of the site's custom UI elements - most obviously its search buttons - appear broken. It really needs someone to look at this