Here are a few that contained violations at the time when I checked them out last February:
https://addons.mozilla.org/en-US/firefox/addon/giphy-for-fir...
https://addons.mozilla.org/en-US/firefox/addon/tabliss/
https://addons.mozilla.org/en-US/firefox/addon/youtube-subsc...
The official policies can be found here:
https://extensionworkshop.com/documentation/publish/add-on-p...
Some highlights of commonly "forgotten" rules:
> No Surprises
> Add-ons must be self-contained and not load remote code for execution.
> Add-ons must limit data collection to what is necessary for functionality [...] Data includes all information the add-on collects, regardless of the manner.
> Collecting, or facilitating the collection of ancillary information (e.g. any data not required for the add-on’s functionality as stated in the description) is prohibited.
> Modifying web content or facilitating redirects to include affiliate promotion tags is not permitted