Right and there is no reason whatsoever it needs to do that. Just paste the URL into my message as plain text and let it go at that. If the recipient trusts me they will open the link, if not they won't, but there is absolutely no reason Thunderbird has to be looking at it when I'm still in the process of composing the message!

It's bad enough that whoever I use as an email provider will look at that link but at least they may have some justification in trying to stop malware embedded in images. But it is none of Thunderbird's damn business what links I may be thinking about sending to another person.