actually, i found a solution after searching through the mozillazine.org forums.

the order of the MIME types is important to Thunderbird. irritating at best and causes 'broken' emails at worst. the specific link i found to help solve the problem is here

In reply to Re^4: problem w/ MIME::Lite and multipart/alternative emails by geektron
in thread problem w/ MIME::Lite and multipart/alternative emails by geektron

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