Rated 4 out of 5 stars
I've used this addon for years and haven't had any stability issues with it. I'm using v. 3.5 with Ffox 11.0. I consider it to be one of my core essential addons; it improves browsing THAT much.
Minor issues: I've noticed lately that sometimes pages have to be reloaded to get the CP icon to start appearing next to links. Also with each feature improvement they've made to the addon it has slowed down the appearance of the popups. (This on a 3.6ghz dual-core Mac). Other than these two minor issues it is a great and recommended addon. - Rob