On this subject I happened to experience something funny. Maybe one year ago I installed a chrome extension (from the official chrome store) to provide an RSS icon directly in the URL bar in order to subscribe to RSS streams. So far so good, then I don't know when, maybe months later I noticed I had some random popups with ads, it was very strange since I use adblock, I tried to find where these ads were coming from, with no luck. Then a day, I used the chrome inspector to analyze a web page and I observed something strange, on every pages I visited an external javascript was loaded, long story short, turned out the chrome extension was a kind of spyware loading a javascript on every pages I visited and sometimes rising a popup. Maybe it took me more time to find it because I only use chrome mainly for testing, my main browser being Firefox, but this is the kind of experience who removed the little confidence I had on the chrome store, and extensions in general, I'm very careful now on what extensions I install particularly extensions needing to be loaded on every pages I visit.