How do viruses work in browser extensions?

Question

Information Security

I downloaded this add-on: https://addons.mozilla.org/en/firefox/addon/cachev...
I thought about the question - what is the chance that 69kb of code can send the entire browser history, cookies, cache and saved passwords to someone else's server?
What language are browser add-ons written in, in particular Firefox?
What files do they have access to?
Access to files only inside the browser, or in the computer entirely?
Can they leak data?
Can they upload data?
How to check the safety of these add-ons?
Are add-on directories checked and controlled in any way?
Thanks for the answer.

Answer 1

Add-ons are mostly written in JS.
And, indeed, they have a lot of opportunities regarding your personal data.
Security check - fully examine the code, if closed - check extension reputation and check hash sums