Everyone's signature is the same template; only these four fields change. Fill them in, then copy for Outlook or download the file for Thunderbird. All brand assets →
-- line above every signature. To drop it,
Settings → General → Config Editor, set
mail.identity.default.suppress_signature_separator to true.Paste into the signature editor, never into a message body. Pasting into a message lets Outlook rewrite the markup, and the logo starts growing a little on every reply. For the same reason, don't drag the logo to resize it: it has to stay 180 × 36.
Classic Outlook attaches the logo unless you tell it not to.
It embeds signature images as attachments by default, even when the source
is a URL. Set Send Pictures With Document (DWORD) to 0
under HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\Mail
and restart Outlook. New Outlook, the web version and Thunderbird already keep the URL.