To install your email signature in Thunderbird for Windows, follow these steps:
Click on the 'Download Signature' button from the Install Signature popup to download your signature file.
In Thunderbird menu, go to Options > Account Settings.
From the left menu select your email account.
In the 'Settings', select the checkbox that says 'Attach the signature from a file instead (text, HTML, or image):' and then click the 'Choose…' button.
Locate the email signature file (.htm) you downloaded in Step 1 and hit 'OK' to save the changes.
That’s it, compose a new email to test your signature.