Useful enough - could be more configurable. Rated 5 out of 5 stars
First off, ignore all the reviewers rating it low and complaining about its accuracy. Task Manager is a joke as far as accuracy. The numbers I'm seeing line up extremely closely with Process Explorer. (A tool which isn't a joke)
I like this addon. For whatever reason, Firefox leaks up past 2GB within a few hours - but this addon serves as a visual reminder to restart it occasionally.
IMO the main things that would make it better are numbers over the top-icon and the option to remove/hide the status-bar numbers. Being able to turn on/off the restart confirmation (individually) would also be nice.