Each email account can have a signature. A signature file is typically an HTML document that can easily be created using Thunderbird. First, to create the actual signature HTML code, follow these steps:
Create a new email message.
Create your signature in the body of the message. Anything valid in HTML can be used, such as images and character formatting.
Select the signature text, and images if you have any.
In the Compose window, select HTML under Insert.
The HTML code that creates your signature will be in the displayed window. Select and copy this text to the Clipboard.
Open Notepad, WordPad, or any other plain-text editor, and paste the HTML from the Clipboard into Notepad (or whichever editor you're using).
Save this file as your signature file. I named my file signature.html. The name choice is yours, though; it is not critical.
Close Notepad and the Compose window.
The next step is to tell Thunderbird to use your new signature. In Thunderbird's Tools menu, select Account Settings. Next, click the account with which you want the signature to be associated. (Each account can have only one signature.) In the Account Settings section, check the box labeled Attach This Signature. After selecting this option, click the Choose button to select the file you saved in step 7 of the previous steps.
If you absolutely must have more than one signature for an account, it can be done like so:
Click Account Settings in the Tools menu.
Select the account to which you want to add another signature. Click the Manage Identities button.
Add a new identity, or select an existing one.
In the Identity Edit or Identity Add window, provide the information (all of which, excluding the name and signature, can be identical to another identity).
Add a signature to the identity being edited or added, and then save the identity.
Now, when you create an email message you can choose which identity is to be used in the From field. You can even switch identities anytime before sending the message.