Danger of add ons for your browser

Actually according to my browser task only as a medium for finding information on the internet, but in some cases increase the need for a browser. Especially me as a blogger. Then made was add on /extension to the browser, but also risky when it would install the add-on because it appears warning that the add-ons can read the privacy on my computer, although the provider of the add on is already in the verification. The question is how far my privacy could be read by the add-ons?
