Version 19.2 of ExQuilla has now been released, supporting Thunderbird versions 17 - 19. Existing users who downloaded ExQuilla from this site should be updated automatically soon. Users who downloaded from the Mozilla addons site will need to wait a couple of weeks until the review is complete there, or you may download the current revision of ExQuilla from this link:
This is primarily a bugfix release. Unfortunately there were a lot of problems with versions 18.0 and 19.0 with message sending. I believe those problems have been resolved in this release.