Where can I find the questions I have starred?
You can find them in your profile on the activities page. On the mobile site, you must scroll down and change the drop down menu to favourites.
marking answers as favorite
No it is not possible to favorite an answer in SE. You can only favorite a question. Maybe you can change this to a feature request, though I doubt it'll be accepted or implemented in the near future. ...
Favorite question star shows undo option even when I did not favorite it
Strictly speaking, the text "This is a favorite question (click again to undo)" is perhaps unambiguous. But depending on how one happens to read it, it may take considerable effort to see what it ...
Only top scored, non community-wiki answers of a minimum length are eligible