in Mozilla/5.0 (Macintosh; U; PPC; en-US; rv:1.0rc1) Gecko/20020417, improper execution of contextual menu occurs while dragging scrollbar slider of the content pane of the mail window. It can be ignored - I don't know what happens if one chooses one of its items instead of continuing the original operation (moot - since it shouldn't appear at all).
This is (yet another!) dupe of bug 74688. Unfortunately it doesn't look like it will be fixed for 1.0. *** This bug has been marked as a duplicate of 74688 ***