Click Preferences-> Account Settings
Click "Composition & Addressing" under your email account in the left pane
Uncheck Compose messeges in HTML
Click Ok
Click "Composition & Addressing" under your email account in the left pane
Uncheck Compose messeges in HTML
Click Ok
Restart Thunderbird
You could also ref: http://www.justanswer.com/computer/2elgc-delete-blue-red-vertical-lines-appear-when.html