Thunderbird

Attach your HTML signature

1. Finish your signature in Sigxel and choose Download HTML.
2. Save the file somewhere permanent on your computer.
3. In Thunderbird, open Account Settings and select the email account.
4. Enable Attach the signature from a file instead and select the downloaded .html file.
5. Compose an HTML message and send a test email.

The signature text field expects plain text or HTML source when Use HTML is enabled. Pasting a rendered signature there is not the same as importing its HTML file.

Keep the file at the selected path. Plain-text messages cannot preserve the designed layout. Images need public URLs that recipients can access.

Source: Mozilla's Thunderbird signature guide.