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Guide · Office and email

How to add a signature with an image or logo in Thunderbird

By Blink Brief · Updated  · How pages are made

In Thunderbird, open Account Settings from the ≡ menu, click your email address on the left, and find Signature text: on the account's page. Tick Use HTML (e.g., <b>bold</b>) and paste HTML whose <img> loads your logo from a public https:// link. Or tick Attach the signature from a file instead (text, HTML, or image): and choose a file. The builder below writes the HTML for the first route, so you don't have to.

Short answer

  • Where: ≡ › Account Settings › your address › Signature text:
  • With a linked logo: tick "Use HTML", paste the HTML, close the tab. The logo must be at a public https:// link.
  • From a file: tick "Attach the signature from a file instead" and choose the file.

Where the signature box is in Thunderbird 159 (and 153 ESR)

Thunderbird 159.0 and 153.6.0esr were both released on October 27, 2026, and neither release note lists a change to signatures. The labels below are Thunderbird's own English words, from its source code:

  1. Open Account Settings: from the ≡ menu at the top right, or in the menu bar under Tools on Windows and Mac and under Edit on Linux.
  2. On the left, click the email address you send from (not its Server Settings or other pages below it).
  3. The account's page shows your name and address at the top, and under them the Signature text: box.

Each account has its own box. If you send from more than one address, paste the signature into each.

With Use HTML (e.g., <b>bold</b>) ticked, Thunderbird reads what you type in the Signature text box as HTML, so the box holds code, not a formatted signature. A logo goes in as an <img> tag whose src is the address of the image:

  1. Put your logo where it opens with a public https:// link, such as your website (right-click the logo on your site and copy the image address).
  2. Make the HTML with the builder further down and press Copy HTML.
  3. In Account Settings, tick Use HTML, click in the Signature text box, delete what's there, and paste.
  4. Write a new message to yourself to see the signature under it.

The image isn't attached. Thunderbird's sending code (read 2026-11-04, on its development branch) leaves an image at an https:// link as a link unless a hidden setting, mail.compose.attach_http_images, is turned on; it's off by default. Each recipient's app then loads the logo from the link when they open the email. That keeps the email small and free of attachments, and it's why the link must work for anyone, with no sign-in.

Route 2: a signature file

Under the box, Attach the signature from a file instead (text, HTML, or image): makes Thunderbird take the signature from a file on your computer, which you pick with the button beside it. The label names the three kinds of file it takes.

People choose a file when they keep one signature for several accounts or edit it in an HTML editor. What happens to the picture depends on where it lives, not on the box you tick. The same sending code embeds a picture from your computer in the email as an attached part, so it shows even for readers who block remote images, but it travels as an attachment. A picture at an https:// link stays a link, as in route 1.

We couldn't read Mozilla's support article on signatures (its site answered with a challenge page on 2026-11-04) and haven't yet sent a test from Thunderbird 159, so send yourself one to Gmail or Outlook.com before you rely on either route.

Which route for which recipients

  • Clients who filter attachments, or mail read on phones: the linked logo (route 1). Nothing is attached and the logo has a fixed width.
  • Readers who block remote images: they see no linked logo until they allow images; a picture from your computer, embedded in the email, shows for them but arrives as an attachment. Classic Outlook for Windows, for one, blocks picture downloads by default. Keep your name, phone and website as text, so the signature works without the logo.
  • One signature in several accounts: a file (route 2) can be chosen in each account; or paste the same HTML into each box.

Make the HTML without writing it by hand

The builder writes the signature as a table with inline styles in Arial, the HTML that email apps display most reliably, with your logo as a linked image at the width you choose. Copy HTML copies the code for Thunderbird's box.

Make the HTML for your Thunderbird signature

To write the HTML by hand, paste this into the Signature text box with "Use HTML" ticked, and replace the example details and the logo link with yours:

<table cellpadding="0" cellspacing="0" border="0" style="font-family:Arial,Helvetica,sans-serif;font-size:14px;line-height:20px;color:#333333;"><tr>
<td style="padding:0 16px 0 0;vertical-align:top;"><img src="https://northwind.com/logo.png" width="80" alt="Northwind Studio" style="display:block;width:80px;height:auto;border:0;"></td>
<td style="padding:0 0 0 16px;border-left:3px solid #0f766e;vertical-align:top;">
<b style="font-size:16px;color:#111111;">Alex Morgan</b><br>Head of Sales, Northwind Studio<br>
+1 555 010 0199<br><a href="https://northwind.com" style="color:#0f766e;text-decoration:none;">northwind.com</a>
</td></tr></table>

Paste the code with Use HTML ticked. If you paste it with the box unticked, recipients get the code as text.

When the image doesn't show: remote content, alt text, the size

  • Missing for everyone: the link isn't public. Open it in a private browser window: it should show only the image, with no sign-in page. Links to Google Drive, Dropbox or OneDrive pages usually open a page, not the image.
  • Missing for some readers: their app blocks remote content. They see the image's alt text, which the builder sets to your company or name, until they load images. Nothing in your signature can change that.
  • Missing in Thunderbird itself: Thunderbird may be holding back remote content in the message you're reading. Look for its notice above the message and allow it.
  • Too big or blurry: the width in the HTML decides the size. Gmail's help gives 100 pixels high as the most for signature images; keep the logo file about twice the width you show it at so it stays sharp.
  • Code shows instead of a signature: "Use HTML" isn't ticked.

More on each symptom, app by app: why a signature logo isn't showing. On a Mac with Apple Mail too? See adding a logo to an Apple Mail signature. For a whole team's signatures, email signature tools compared.

Questions

How do I add an image to my Thunderbird signature?

Open Account Settings (the ≡ menu, or Tools › Account Settings), click the account's address, tick "Use HTML (e.g., <b>bold</b>)" under "Signature text:", and paste HTML with an <img> tag whose src is a public https:// link to your logo. The builder on this page makes that HTML.

Why is my Thunderbird signature image not showing?

If the HTML points to an image on your computer, or to a link that needs a sign-in, recipients can't load it. Use a public https:// link to a PNG or JPG. If the link is public, the recipient's app may be blocking remote images until they allow them, which is why your details should be text.

Can Thunderbird use an HTML signature?

Yes. Tick "Use HTML (e.g., <b>bold</b>)" under the signature box and the box is read as HTML. The other option, "Attach the signature from a file instead (text, HTML, or image):", takes the signature from a file on your computer.

Do I need a separate signature for each account?

Yes. The signature box is on each account's own page in Account Settings, so paste it into each account you send from.

Sources

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