Thunderbird, chat extension, what about notification
FiltaQuilla is an add-on that enables the user to properly and precisely configure notifications, based on built-in filters or enhanced filters. Maybe one could try to extend its functionality to filter/notify chat messages. I don't have a full answer here, but it might be a good start. I would not be surprised if the developer of this add-on would make something for chat.
This I currently use this extension to deal with this issue.