Frequently sent email messages can be conveniently saved as templates by selecting "Save" and then "Template". The email message will be saved in the "Templates" folder attached to the email account and can be re-used by right-clicking on the message header and selecting "Edit as New ...".
Preferences (for example, font style, font size, etc) these can be modified by selecting "Tools" and then "Options".
Thunderbird Email Signature
An email signature can be created by selecting "Tools" and then "Account Settings" and then pointing Thunderbird to the desired signature file by checking the box and adding the path to the file (plain text, html, etc) in the box with header "Add the signature from a file instead (text, HTML, or image)" by clicking "Choose ..." and selecting the desired signature file.
These should be added to the Gmail contact database whenever possible and source photos stored in the Dropbox.