Thunderbird has a configurable option for auto-completion of email addressses.
What’s auto-completion? Create a new email in Thunderbird and then start typing the email address in an address field. Thunderbird will automatically match the letters you type against its database of email addresses its created from your email activity.
Where can you find the configuration options?
From Thunderbird’s menu bar (you might have to press and release the Alt key to display the menu bar), select Tools > Options. From the Options dialog box, select Composition and then the Addressing tab.
You are able to turn on or off the auto-completion. Just uncheck both entries under "When addressing messages, look for matching entries in".
If you don’t want Thunderbird to keep a database of the email addresses of outgoing emails, you can uncheck the box beside "Automatically add outgoing e-mail addresses to my: Collected Addresses."
Of course, after making your selections, either click OK to save the changes or Cancel to close the Options dialog box without saving.