By default every time you reply to a message, Thunderbird adds a colored line down the sides to ‘frame’ each particular reply.  This can get rather distracting after awhile. Plus it makes the overall length of the email a lot longer as the message content area gets narrower with each reply.

To eliminate this, add these lines of code into your userContent.css file [directions] located within the chrome folder inside your profile folder:

/*Remove reply lines*/
 blockquote[type=cite] {
 padding-left: 0 ! important;
 border-left: none ! important;
 border-right: none ! important;
 }