A working mailbox login does not prove that its signature resources moved with the computer. Thunderbird can store signature text in the account settings or use a separate file. Identify which arrangement the account uses before rebuilding a signature from an old email.

Trace the resource used by the account

Open the selected account's signature settings. Check whether the text is entered directly or the option to attach a signature from a file is selected. For a file-based signature, inspect the configured path and confirm that the approved file is available on the new computer.

Multiple accounts have separate signature configurations. Compare the affected account with another only to identify the difference; do not copy personal contact details into a business identity just because that signature still displays correctly.

If an HTML signature contains a logo, determine whether it is attached to the message or loaded from a website. A remote image depends on connectivity and the recipient's content settings. The absence of that logo does not by itself show that the signature text or mailbox is broken.

Review a draft in the formats you use

Create an unsent sample draft and inspect the name, telephone number, business address and links. Compare it with the current approved wording, not merely with a historic message that may contain an old role or location. Avoid placing real customer information in the sample.

Check plain-text behaviour as well as HTML if both formats are used. Important contact details should remain understandable without a decorative image. For replies, review whether the signature appears above or below quoted text according to the account's composition preference.

Save the sample for review and record the approved signature file's location in the device handover. A successful draft check demonstrates the local configuration; recipient rendering remains a separate observation when a normal authorised message is later exchanged. Keep the reusable signature resource with the account's documented setup so the next move does not depend on finding a matching old email.

Source: Thunderbird documentation.