There is one company's email that never arrives in my box. They told me to bring their email adress in the safe emailadress in my account. But I don't know how

If the email is never downloaded into Thunderbird, you need to find out if that email is arriving on the server. If you use a POP mail account then it can only access the server Inbox and download from server Inbox. If the server has filtered the email into the server Spam folder, then Thunderbird cannot access nor download it. So first you need to find out if that email has arrived on server and whether it is in the Spam folder. Open a browser like Mozilla Firefox and logon to the webmail account of your ISP via their website. Is the email in the server Spam folder ? If yes, then select the email, and see if there is a 'Not Spam' button and move the email into the server 'Inbox' . Then in Thunderbird click on 'Get Messages' button to access server Inbox and download the email.