Marking senders as not spam

First, there is no quick solution, no 'white list' in Thunderbird. This URL has info on available options:

https://blog.thunderbird.net/2024/09/thunderbird-and-spam/    additionally, you might check your email provider's options, as many have white lists and spam filters that work at the server level prior to the messages being available to Thunderbird. There are also some third-party products, such as Mailwasher, that can do wonders (that's what I use). You can also create a filter for known senders whose messages go to spam. Good luck.