To add a signature to your email when writing or replying to emails, follow these steps:
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Click on the Start menu and select My Documents.

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In the My Documents window, right-click on an empty part of the window, select New from the context menu, and then click on Text Document.
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Insert your signature text, save the file and close it. You can rename the file to whatever you like. You should give it a name that will identify the file later - e.g. email_signature.txt.
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Open up Thunderbird.
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Select Tools from the menu in Thunderbird and click on Account Settings...

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Click on your default profile on the left, then tick the box on the right where it says "Attach the signature:".
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Press the Choose... button beside the checkbox, navigate to the My Documents folder, and double click on the new text file you creating earlier in steps 2 and 3.
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Next, click on Composition & Addressing under your profile on the left.

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From the options on the right, choose Automatically quote the original message when replying Then, start my reply above the quote and place my signature below my reply (above the text).
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Press the OK button to save your changes.