After a little trial and error, I can confirm Thunderbird/Lightning is working with Caldav and SM 14 again, but with a change to the path as mentioned above to add "default/" to the path. Thus the correct path (as shown above) is:
For those like myself who use multiple calendars, I have made this work in Thunderbird/Lightning with SmarterMail by creating different email accounts for each calendar, and then using this format in the path for each:
Please note you need the email accounts of each user account that is used for each calendar created in Thunderbird 1st, and to select them in the setup process. Also please note this also works for SmarterMail before version 14, but without the "default/" at the end (but still end the url with a "/"). For those that are curious, the %40 is not a typo, but is apparently how the @ sign is used in Caldav without issues.
I am more than a little disappointed SmarterTools did not document all of this, and indeed removed the help/KB for Thunderbird instructions which DID work for versions up through v13. I hope they will now put those instructions back for Thunderbird and other users with the additions for v14, and take this a step further and also document the urls for non-default calendars and multiple calendars as well.
In a land of Outlook.com, Google calendars, etc, where all of this stuff just works and is well documented, SmarterMail and those of us who support it cannot afford to have this NOT work.