How to add an email signature in Thunderbird

Thunderbird offers more signature control than most webmail editors, but that control is split across account settings, HTML composition and saved files. You can type a simple signature directly, enter HTML markup or attach an HTML file created in Thunderbird's composer.

This guide shows all three methods and the reply-position setting that prevents a signature from landing beneath an entire quoted conversation.

Choose the right Thunderbird method

Mozilla's current Thunderbird signature documentation describes the text field and file-based workflows. Signatures are configured separately for every account and identity.

Add a plain-text signature

1. Open Thunderbird's menu.
2. Choose Account Settings.
3. Select the email account in the left panel.
4. Enter the sign-off in Signature text.
5. Close settings and open a new message.

Plain text is the most dependable option. It works in both plain-text and HTML messages and cannot lose a logo, font or layout. Use it when a mailbox is primarily for support, technical alerts or low-bandwidth communication.

Enter an HTML signature directly

In the same account settings, select Use HTML beneath the signature text field. Enter simple HTML markup, then compose an HTML message to preview it.

Email HTML is not normal website HTML. Use tables for columns, inline styles, absolute HTTPS links and common system fonts. Do not rely on flexbox, CSS Grid, external stylesheets or JavaScript. If a message is sent as plain text, Thunderbird converts the markup to a text representation and images do not appear.

If you created the design in BrandSig, use the file method below when pasting rendered content into the small settings field is inconvenient.

Create and attach an HTML signature file

Thunderbird can use a local text, HTML or image file as the signature source.

1. Open a new message and make sure the formatting toolbar is visible.
2. Build the signature with the normal formatting tools.
3. Choose File > Save As > File.
4. Select HTML files as the type and save it in a permanent folder.
5. Return to Account Settings for the mailbox.
6. Select Attach the signature from a file instead.
7. Choose the HTML file you saved.

Do not leave the file in Downloads if you routinely clean that folder. Thunderbird reads it from the stored location, so moving or renaming it can break the signature.

Add a logo or photo

While composing the HTML signature, use Insert > Image. You can choose a local image or a remote URL, set its display size and add a link. Mozilla also provides an Attach this image to the message option.

An attached image travels with every email and can increase message size. A remote image keeps messages lighter but may be blocked until the recipient allows remote content. In either case, keep the company name as text and provide alternative text for the image.

Use a transparent PNG for a logo, keep it visually modest and test dark mode. The best-practices article has practical width, file-weight and font limits.

Configure multiple accounts and identities

Thunderbird stores signature choices per account and per identity. Repeat the setup for each address that appears in the From menu. This is helpful for aliases, shared mailboxes and multilingual identities, but it also means editing one account does not update the others.

Name and store file-based signatures clearly, for example sales-new-message.html and support-compact.html. Keep the files in a backed-up folder and avoid using two different files that reference the same local logo with conflicting paths.

Put the signature above quoted replies

By default, Thunderbird may place a reply signature beneath the quoted message. To move it where readers expect:

1. Open Account Settings.
2. Select the account, then Composition & Addressing.
3. Set replies to start above the quote.
4. Set and place my signature to below my reply (above the quote).

Configure this for each account or identity. A correct signature file cannot fix the wrong reply-position preference.

Fix common Thunderbird signature problems

HTML shows as code. Enable Use HTML or attach an .html file, and compose the message in HTML mode.

Formatting tools are missing. The current message is plain text. Change the delivery format to HTML before building the signature.

The signature disappeared after moving files. Select the file again in Account Settings or restore it to its previous location.

A local logo is missing for recipients. Insert it through Thunderbird and attach it to the message, or use a public remote URL.

The signature sits at the bottom of the thread. Change Composition & Addressing, not the signature editor.

It works for one address only. Repeat the setup for the other account or identity selected in the From field.

Test both HTML and plain text

Send an HTML message, a plain-text message and a reply to another provider. Verify the logo with remote images blocked, links on a phone and the position relative to quoted content. Thunderbird's flexibility is valuable when the source file, message format and account identity are treated as separate settings.