Menu -> View -> Message Body as -> Plain Text
furthermore, you have to change some values in about config, see this articele in the german TB forum
you need to disable the default setting for "the holding of messages", otherwise TB always downloads the entire mail for offline reading. An attachment is indeed a direct component of the mail and not a separate file.
There should be no filters applied to the message body or attachment.
Menu -> View -> "Display Attachments inline"
must be checked.
In the advanced configuration then you have to Check if
mail.imap.mime_parts_on_demand is set true.
Leave the entry
mail.imap.mime_parts_on_demand_threshold on 30000 (= 30 KB)
because attachments less than 30 KB are rare (eg *.Vcf files, etc.) and do not interfere with the retrieval of mail.
also see: http://kb.mozillazine.org/Entire_message_fetched_when_opening_a_IMAP_message