If you have iMessage, then the files are backed up in ~/Library/Messages/chat.db
This script will process them from a wild collection of .plists to a single html file
It relies on sqlite3 & python (should be factory installs)
git clone https://www.github.com/saylestyler/iExport && cd iExport- run
python iExport.py > your-backup.html - njoy
open your-backup.html -a Firefox