De quebra automática de palavras :
This may sound a bit technical, but Thunderbird is doing exactly what it is supposed to do. When you have a message in plain-text format which doesn't come with flowed format, by definition of the standard, the receiving client is not allowed to rewrap that message to fit the window width. This is only possible if the "format=flowed" attribute is set. Consequently, Thunderbird assigns a "Preformat" style to any quote which comes from a plain-text message which does not have that attribute set. It would receive "Message Body" with format=flowed set.
A workaround for this is to highlight the entire text immediately after clicking on reply (i.e., CTRL+A), then change "Preformat" to "Body Text" in the left-most drop-down menu. This should avoid the issue you observe. In general though, if the sending client formats the e-mail in a fixed way, it's probably correct if Thunderbird treats it this way. In the worst case, there is always Edit > Rewrap, even though it's not optimal either.