(Currently only tested for XUbuntu 18.04, all feedback on other systems welcome!)

Head again to the latest release and download the cb_thunderlink_linux.zip from the assets.

Unzip the cb_thunderlink_linux.zip e.g. to ~/cb_thunderlink. This location must be a fixed location (no network location!) and permanently available.

Open a shell and issue :

> cd ~/cb_thunderlink
> ./cb_thunderlink register

It registers the executable to the add-on and it registers the protocols thunderlink:// and cbthunderlink:// to the operating system.

Remove and reinstall the cb_thunderlink.xpi into Thunderbird and that's it. From now on you should have again clickable (cb)thunderlink's!