Note: Make sure you use Google Chrome or Firefox during installation.
Create your email signature on MySignature.io and follow these simple steps to install it in Thunderbird:
After creating your email signature, click Save signature button under the signature preview.
Or, if you have saved your signature, find your signature on the Dashboard and click Preview and install icon.
Select the Source code in the list of email clients and click Copy source code button.
Then open Mozilla Thunderbird.
Click on your email account name, e.g., firstname.lastname@example.org, and click Account Settings.
In the Account settings window, go to the Signature text section.
To use HTML code, you need to check the Use HTML box.
Now you can paste your copied source code to the Signature text.
Go to the main view of Thunderbird and click Write to create a new email message.
Important note: When you compose a new message, it has many red lines around the layout of the signature, like on the screenshot above. That's okay. Thunderbird adds these red borders to show you HTML tables and cells on which the signature was constructed. The Table itself does not have any pixel border size, so you would not see the Table's construction. However, those are not part of the HTML and should not appear in the sent/received message. Please check the guide. Thunderbird has red borders inside and around the signature for more information.
Viola! Now, every new email, reply, or forward message you compose will have your Thunderbird email signature.