Thunderbird supports HTML signatures. Switch the signature field to HTML, then paste the code or point it at your downloaded file.
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Copy the signature code
Download your HTML signature file from your company signature page. Open it in a plain text editor: Notepad on Windows, or TextEdit set to plain text on Mac (Format → Make Plain Text). Select all and copy the code.
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Open the account signature settings
- Right-click the account name and choose Settings. You can also use the menu button → Account Settings, or Tools → Account Settings.
- Select that account and tick Use HTML under Signature text.
- Open Composition & Addressing for the account and tick Compose messages in HTML format.
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Paste the signature
Paste the code into the Signature text box and close Account Settings.
Alternative: tick Attach the signature from a file and choose the HTML file you downloaded.
Click Write and check the new message.