I'm usually a picky bast*rd but this add-on delivers. I have tried many content tracking add-ons and I can confidently say PageProbe is by far THE BEST of its kind. It is highly configurable and very reliable. I use it for tracking product prices and thanks to it I've been able to save money on my online purchases. Just to mention some of my favorite features, it keeps a history of old values that it can display in graph form, it allows organizing trackers into folders and sub-folders, tracker import and export, and it's cross-platform (Firefox and Chrome). I experienced a bug which the developers resolved within hours of my report, and an update was released the very next day—really impressed. I'd love to see email notifications added in the future, if possible at all. Thank you!