security - How can I be sure that browser plugins are secure?

There are plenty of browser add-ons. I want to know how can I know if these add-ons have good reputation in terms of security? How do I know these are not programs that could harm my PC? Most of the vendors are not house hold names and most of the add-ons are free. What do you suggest I do? Is there a body that certifies these programs in any way? Thanks all.