Thanks for the reply.
I have downloaded the latest version of TEBE from the website. Found the ZIP file and renamed it with .xpi.
Then opened Thunderbird, gone to tools, add-ons, install. I found the file and installed it. Thunderbird wanted to restart, so did.
And nothing. TEBE is not installed, no mention of it in tools or add-ons.
Went to tools, error console but no mention of it.
Point of interest - after renaming the original ZIP file it now contains the extension .xpi.zip