From mozillaZine (Mozilla Wiki):
Suppressing automatically-generated links
It is possible to take some steps to suppress automatically-generated links, but you cannot completely suppress them.
When you write an HTML message, you can suppress automatically-generated links.
Select the text that Thunderbird normally linkifies.
Choose Insert – HTML... In the HTML editor, type anchor tags around the text.
For example, if you type this but you do not want Thunderbird to linkify it:
In the HTML editor, change it to:
If there is already an anchor tag with attributes (class and href, typically),
then remove the attributes to leave the tag looking like the example above.
Note: People who receive your message might still see a link,
if they use mail clients that linkify when they display messages.
For example, Thunderbird users will probably see a link if they choose:
View – Message Body As – Plain Text
If your message is sent using HTML formatting, Mozilla Thunderbird will automatically hyperlink URLs and email addresses.
In the plain text version, the URLs and email addresses will remain untouched, however the recipient's email program will typically turn these addresses into hyperlinks.
If you wish to send the mail in plain text mode, here is a quick tutorial on how to switch between HTML and plain text emails in Thunderbird (notice the difference between HTML and plain text modes for reading (viewing received messages) and sending (composing new messages and sending them)).