Alternatively, you can upload a file that contains your signature. Check Attach the signature from a file instead and then click Choose... to select the file. The file can contain either plain or HTML-formatted text. If you have an HTML-formatted signature, the message recipient must be able to view HTML-formatted messages in their email program. If they have disabled this ability, the signature will be rendered in text format and images will not be displayed.

One way to create a signature file is by using the Thunderbird composer (用打電郵方式去打). As an example, create a new HTML-formatted message in Thunderbird (File > New > Message). Make sure that the formatting toolbar is displayed.

(If it is not displayed, you are composing a message formatted in text, not HTML. To change to HTML, select Options > Delivery Format > Rich Text (HTML) Only.)

1.    Compose and format your signature as desired. Note that numerous formatting functions are available from the Insert and Format menus.

2.    Click File > Save As > File. Make sure "HTML files" is selected, and then specify a file name and click Save.

3.    Close the message window and discard the message without saving.

4.    Open the Account Settings and select the email account in the panel on the left.

5.    Check Attach the signature from a file, click Choose and navigate to the file you created.

Including image files in signatures

To include an image file from your local computer in a signature, follow the steps above to create an HTML signature. When you are composing the signature contents, though, use the Insert > Image menu option to specify the desired image.

In addition to selecting the image file, use this dialog to configure other aspects of the image, such as the size, a URL link, its position with regards to the text, etc.

You can also specify an image located on a web server as part of your attachment. Simply specify the image URL in the field where you would otherwise specify the file name. If you check "Attach this image to the message" the image will be included as an attachment. If you do not attach the image, message recipients must have an internet connection to view the image. Also, keep in mind that for security reasons many people configure their email programs to block remote content, which would prevent the image from displaying unless it was attached to the message.

 

For detail help you may visit the under link for full helping.

https://support.mozilla.org/en-US/kb/signatures