There is a feedback and ideas forum here connect.mozilla.org you can also access the link from the help menu in Thunderbird. I really do not see your request getting much support however. There is little difference in a menu with a picture that deletes and a word on a menu that deletes. The argument will be that you have access to a delete option using the mouse without additional clicks All that has changed is the visual representation of the action. Regardless of your preferences in that regard, we all have to accept change.