Thunderbird running Imap is merging a series of some e-mails. Merged data appears to be in hexidecimal.
My thunderbird running in Imap mode under win 7 32 bit (102.0b4 (32-bit)) is merging groups of messages under one e-mail title entry. Merged messages under the initial message appear to be in hexidecimal or at least binary. I don't see a pattern in the messages being merged.
Wšykne wótegrona (1)
More likely it is unformatted HTML.
Try right clicking the folder,selecting properties and then the repair button.