No, they are not. But if they are open-source (some of them are) you can ask the developer to give you the code (if it's not already published on his website or some repository and you can compile it by yourself; or just use if it's something for the web etc.) and you or somebody else can help you analyse the code and what it does. There is no other way to be 100% sure unless you use open source or in the ideal case free software, where you can see the code. Never trust 100% to the proprietary software, because nobody except the authors have reviewed the code (except maybe some reverse engineers ;). There were occurences of "bad" borwser addons e.g.  http://www.pcadvisor.co.uk/news/internet/3212049/firefox-browser-add-ons-contained-malware/

No, they are not. But if they are open-source (some of them are) you can ask the developer to give you the code (if it's not already published on his website or some repository and you can compile it by yourself; or just use if it's something for the web etc.) and you or somebody else can help you analyse the code and what it does. There is no other way to be 100% sure unless you use open source or in the ideal case free software, where you can see the code. Never trust 100% to the proprietary software, because nobody except the authors have reviewed the code (except maybe some reverse engineers ;). There were occurences of "bad" borwser addons e.g. 

http://www.pcadvisor.co.uk/news/internet/3212049/firefox-browser-add-ons-contained-malware/

or more recently

http://thenextweb.com/facebook/2013/05/13/microsoft-warns-users-of-new-malicious-chrome-extension-and-firefox-add-on-that-hijack-facebook-accounts/

But realistically speaking, we are all forced to use these addons if we want to keep the speed/efficiency with others. So in the end, you need to take some risk and install them, unfortunatelly.