Many add-ons have somewhat spookiy authorisation requirements, such as “access all of your activity”. In many cases this is justified by it’s function, and of course there isn’t any problem with it as long as we’re sure all this data stays on your computer and isn’t shared with any remote server. How are we sure of that tho? Is there an easy way to check for each add-on ?
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.