How to add an email signature in Thunderbird

Before you start: get the HTML

Thunderbird's signature box is a plain text area. With Use HTML ticked it treats what you paste as HTML source, not as formatted text, so you need the code rather than a rich-text copy. The free signature generator has a Copy HTML button for exactly this, and a Download .htm button if you would rather attach a file.

Paste the HTML source

1. Open the application menu (three lines) and choose Account Settings.
2. Select the account in the left column. The main page shows the identity: name, email and Signature text.
3. Tick Use HTML.
4. Click into Signature text and paste the HTML source.
5. Close the tab. Thunderbird saves as you go.

Start a new message to check the result. If you see raw tags in the message, Use HTML was not ticked.

Or attach the signature from a file

For a longer signature, or one you share with colleagues, keep it as a file so edits happen in one place.

1. Save the signature as an .html or .htm file (the generator's Download .htm does this).
2. In Account Settings, tick Attach the signature from a file instead (text, HTML, or image).
3. Click Choose… and select the file.

Thunderbird reads the file every time you compose, so replacing it updates the signature everywhere it is used.

Different signatures per address

Each identity has its own signature. If you send from more than one address on the same account, click Manage Identities… at the bottom of the account page, select the identity and set its signature there. Thunderbird switches signature when you change the From address while composing.

Common problems

Two dashes appear above the signature

Thunderbird inserts the standard -- separator. To remove it, open Settings → General, scroll to Config Editor, search for mail.identity.default.suppress_signature_separator and set it to true.

The signature sits below the quoted text on replies

In Account Settings → Composition & Addressing, set Start my reply above the quote and then place my signature below my reply (above the quote).

No signature on replies or forwards

Same page: tick Include signature for replies and Include signature for forwards. They are separate options.

The logo is a broken image, or arrives as an attachment

Reference the logo by a public HTTPS address. A local file path works on your machine and nowhere else, and an embedded image is sent as an attachment on every message. Recipients also need to allow remote content to see it, which is the trade-off every hosted logo makes.

Fonts look wrong

Thunderbird honours the inline styles in the HTML but ignores stylesheets and web fonts, like every other client. Use Arial, Helvetica, Verdana or Georgia with a fallback.

Setting signatures for a whole company

Thunderbird has no central signature management: each install keeps its own file or text. If your mail runs on Microsoft 365 or Google Workspace, UnifySig adds the signature server-side as mail leaves the tenant, so Thunderbird users get the same signature as everyone else without configuring anything. Leave the Thunderbird signature empty in that case so it is not added twice.