Open your HTML email signature file in your web browser.
Press Cmd + A to select the entire email signature, then press Cmd + C to copy it.
IMPORTANT: Do not use your mouse to select the area, only use Cmd + A as you may miss out on grabbing some of the code.
Open Mailbird and click the Settings icon at the bottom left of the email client.
Select the Signatures icon at the top of the window and click Add Signature.
Enter a name for your email signature, select the default email account from the dropdown below, then click inside the signature field and press Cmd + V to paste your copied email signature.
Compose a new email and your email signature will now automatically insert when sending an email from Mailbird on macOS.