quoted text in reply page has lines too long

when i reply to a message, the original message gets quoted (lines start with a ">") , which is fine. The problem is that the lines are too long, they do not fit the screen. I do not understand why: when i type a text, thunderbird correctly cuts the lines -- it never goes off the screen. And also when i desplay a message the text never goes off the screen. In short: i can reply to a message and when i compose my text the original text in the quote goes off the screen but then later when i display the email (assuming the email for instance was Bcc-ed to me) the quoted text that before would go

off the screen can now be seen correctly. 

Thanks for your help.