As an 'extension' to earlier questions on this topic (for instance here and here),

• What makes Firefox extensions (appear) more secure than Chrome extensions?
• Do they just appear to be more secure, or are they really?

See also:

• Why do Chrome extensions need access to 'all my data' and 'browsing activity'?
• Worst case scenario, what can a Chrome extension do with "Your data on all websites" and "Your tabs and browsing activity"?
• Are official browser add-ons really safe?
• http://lifehacker.com/5990769/why-do-chrome-extensions-need-to-access-all-my-data
• https://support.google.com/chrome_webstore/answer/186213?hl=en-GB