Email signature generator › Thunderbird

How to add an HTML signature in Thunderbird

Thunderbird is one of the few desktop clients that will take a raw HTML file as your signature and re-read it every time you compose. That makes it the easiest client to maintain — and the easiest to get subtly wrong.

The two methods, and why the second is better

Thunderbird stores signatures per account, under Account Settings (right-click the account in the folder pane → Settings). There you have two boxes that do almost the same thing:

1. Signature text, with the checkbox Use HTML ticked. Whatever you paste here is stored inside the profile. It works, but editing it later means coming back into this dialog and fighting a single-line textarea.
2. Attach the signature from a file, pointing at an .html file on disk. Thunderbird reads that file each time you open a compose window.

The second is better for one practical reason: you can open the file in a text editor, change one line, save, and the next email you write already has the new version. No dialog, no re-paste, no chance of the editor rewriting your markup on the way in.

The trap in method one

If you tick Use HTML and paste markup into the text box, Thunderbird treats what you typed as HTML source. If you forget to tick it, it treats the same text as literal characters and your recipients receive <table> and <td> as visible text. This is the single most common Thunderbird signature complaint, and the fix is one checkbox.

Going the other way is subtler. If you tick Use HTML and paste formatted content copied from a web page or a word processor, you get whatever markup that source carried — often <div> soup with inherited CSS that will not survive in Outlook. Paste source, not appearance.

Images: the part that behaves differently here

Thunderbird gives you a real choice that most clients do not. If your signature file references an image with a relative path or a file:// URL, Thunderbird will attach the image to the message when you send. The recipient sees it inline, and it does not depend on a server staying up — but it adds weight to every message and shows up as an attachment paperclip in some clients.

If you reference an image with an https:// URL, nothing is attached and the recipient's client fetches it — which means it will be blocked until they choose to load remote content.

Neither is wrong. Attached is more reliable and heavier; hosted is lighter and depends on the recipient allowing remote images. For a logo under 20 KB, attaching is usually the better trade.

Plain text and the second signature

Thunderbird can send plain-text messages, and some correspondents and mailing lists still expect it. When it does, your HTML signature is converted by stripping tags — which turns a neat table into a run of words. If you write to lists, keep a plain-text version of your signature and switch the account's composition default rather than letting the converter guess.

Per-account, not global

Every signature setting in Thunderbird belongs to one account. If you have three addresses in the same profile, you set three signatures, and changing the sender address in the compose window swaps the signature with it. That is usually what you want, and it surprises people who expected one global setting.

Frequently asked questions