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Firefox (Chrome) should make it very easy for anyone to audit what extension/plugin are doing.

If url, div, cookies and any other info are collected, what are they?

What server connections are made by the extension, IP, Name, contents of info transmitted?

All the GUI, collection system should be in place as part of JS dev/debug tools already. Just customized it a bit so any tech savvy users can check the audit logs and enable more logging for a plugin if needed.

If an user spots something not right, it is also easy to out the "plugin/extension" on a public forum.