Step 1:
Open Mailbird. Once you are at the main screen, click the options button on the top left, and click on Settings.
Step 2:
From the left menu, click on Identities. Find your email account on the right, select it and click Edit.
Step 3:
Paste your new email signature into the Signature section, and click OK.
Step 4:
Your new email signature is now installed! Test it out by composing a new email to make sure it looks correct.