Well, any and all browser addons can do anything, based on the permissions they need, and what you give them. I wouldn't go so far, as to say to not get any, but most of the major extensions have to go through the occasional code check, not to mention that the browser stores allow for reporting.
Biggest thing is that if you are doing something in the gray area, and downloading extensions that don't have a high user approval rate and download amount, then you might be getting something that might not have been written for your benefit. Doesn't mean all newer and under used addons are bad, but there is always that possibility.