Which is it? Spam, scam or phishing? Thunderbird scam detection has one adjustment. On or off. Thunderbird does not mark messages Spam. Check with your provider or any spam detection software you are running.

Firefox is an Internet browser and has nothing to do with the email client Thunderbird.

Thank you for your answer! It's mostly scam, sometimes phishing. But it clearly says 'Thunderbird thinks this is' or 'Thunderbird has marked this', so that's why I presumed this is in fact generated by Thunderbird. I've seen the adjustment you mention, but I don't want to turn off scam detection entirely, so I guess I'll just have to deal with this...

Here is the info on Scam detection. Maybe the sender can fix whatever they have added, most likely to their signature, that triggers it. https://support.mozilla.org/en-US/kb/thunderbirds-scam-detection

Thanks again