I have been using this extension for a while and I like that it works right out of the box without a bunch of configuring. I also like that it has filters like price drop alerts and such, although I haven't gotten around to using them.
-1 star for: "This add-on is not actively monitored for security by Mozilla. Make sure you trust it before installing"
I'm not sure how you're supposed to be able o quickly research and monitor the security of an extension.