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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
I think it's "access your data" permissions that are the ones to be wary of, due to the explanation here. Defending against this, I'm not really sure. Someone who knows more should chime in, but maybe a software firewall like Little Snitch/OpenSnitch that will let you approve/deny every connection. (This will probably get fatiguing fast.)