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How to check that an add-on doesn't spy on you ?
(sh.itjust.works)
A place to discuss the news and latest developments on the open-source browser Firefox
Easiest way is to use packet sniffing. but they could be set-up to only send data at random intervals, which could be hours, days or weeks. so you're best bet is go through the source code by extracting the extension file and look for any links, domains or mangled code which may be encoded with something malicious.