No one wants to have a go here. I don't want to either really. But what you are seeing is what is talled a web bug. It is a single pixel image used by commercial mailers to track your activities in the email when and where is is read being the most ovvious but also how many times you open it as it is fetched everytime you look at the email.

Why it is opening in your browser can at this point only be conjecture. But I am guessing it may be like the old twitter icons and been included in the message as a remote image, but the link is to a script that serves the image, not to the image. Thunderbird does not accept output from scripts so the browser open to get the image. You might want to consider blocking remote images.

A script can grab a whole lot more fingerprinting information that simply serving an image so there are privacy implications for these things

No one wants to have a go here. I don't want to either really.

I'm not sure why 'no one want to have a go'. Is this something I should be ashamed of? As mentioned in the original query, the source of these problem messages is a major UK supermarket (probably the biggest), and I have been receiving these messages from them for years, without any problems until recently. Some are 'Special Offers', but most of them are information about forthcoming deliveries.

But what you are seeing is what is talled a web bug. It is a single pixel image used by commercial mailers to track your activities in the email when and where is is read being the most ovvious but also how many times you open it as it is fetched everytime you look at the email. Why it is opening in your browser can at this point only be conjecture. But I am guessing it may be like the old twitter icons and been included in the message as a remote image, but the link is to a script that serves the image, not to the image. Thunderbird does not accept output from scripts so the browser open to get the image. You might want to consider blocking remote images.

OK. If I understand this correctly, Tesco are monitoring what and when I read these messages; I don't really have a problem with that, but the implication is that Thunderbird is not handling these scripts correctly, especially since I don't have the same problem if I use a different mail client.

I have tried clicking on messages sent to me some time ago and the problem occurs with them too. However, I have only noticed this problem recently (within the last week or two), so I would say that something must have changed in Thunderbird since then.