A beta version that works under TB78 is available, although not all features are back. In particular, there is only one border style at the moment, and no translations.
If you encounter any issues, please let me know on the issue tracker.
If you have Thundebird configured with multiple accounts or identities, you might have sent emails from the wrong one once or twice. This add-on tries to prevent that mistake by adding a colorful border around the "New message" window. You can configure each identity to have a different color.
This add-on builds on the work of BorderColors and BorderColors-GT. It fixes a couple of bugs and adds multiple border styles: partial borders, gradients, etc.
Remember to uninstall other BorderColors add-ons if you are using this version!
Source code and issue tracking on GitHub.