Thank you for these analogies, I didn’t think of adblockers and stuff and you’re right, this is a common feature…
I didn’t want to write my question in an “ambiguous” way, just did not want to mention any details since this seemed to be a question in general to me.
So no, I think my addon would not make the site owners unhappy, it just adds features like sorting some list stuff. But yeah, copyright or trademark infringement is a thing, because for sure I would have to mention the specific site/service the addon is made for in order to promote/describe it. Are there any precedents for DMCA-takedowns on addons forged to work with a specific third party service or any recommendations how to handle name, description etc.
Just to be clear, I don’t wan’t to do any legal limbo with my extension or the third party site, I just wan’t to make some users happy who face the same problems like me using the service. I’d originally build it for myself and some friends, just wanted to offer my work to other users.