Add your signature to Thunderbird

⏱ About 90 seconds

1. In the OnceSig editor, click “Copy signature” (or “Download .htm” for the file method).
2. In Thunderbird, open the application menu → “Account Settings”.
3. Select your email account in the left sidebar.
4. On the main settings page, find “Signature text” and check “Use HTML (e.g., bold)”.
5. Paste your signature into the text box with Ctrl+V.
6. Alternative: check “Attach the signature from a file instead” and browse to the downloaded OnceSig.htm.
7. Close the tab — settings save automatically. Compose a new message to confirm.

Good to know

• The “Use HTML” checkbox is per-account. If your signature shows raw code like <table>, the checkbox isn't ticked for that account.
• Plain-text recipients: when you send a plain-text message, Thunderbird falls back to a text version — recipients never see broken markup.