Download the latest TEBE by clicking this link
. In the download window that Fx brings up, the file name to save is listed as "firstname.lastname@example.org
". Remove the ".zip" at the end and save the file as "email@example.com
In Thunderbird, click Tools > Add-ons > ("gear" icon) > Install Add-on from file...
. Navigate to where you saved firstname.lastname@example.org
, double-click on the name and follow the prompts to finish the installation.