Did you check your Spam/Junk/Bulk folder? If your accounts are POP, you'll have to check via webmail, as POP only downloads the Inbox.

I contacted Thunderbird first because of the mystery of the empty email notification box. I get a window that pops onto the lower right hand of my monitor when a new email arrives. It most often gives the name of sender and the title. I am somewhat new to "Bitdefender" and I'm not sure I know what all it defends me from. But, I have also been getting incoming email notification windows that are devoid of any other information. As I wrote, it is an empty box. Up until knowing that I have not been getting PayPal invoices, I was thinking it was spam that was being intercepted. I don't know. Can you tell me that, since it is a Thunderbird notification, it is not some censorship on your part?

Are these notifications presented by the built-in option or by the Windows system notifications? The choice is in Settings/General/Incoming Mails.

TB doesn't censor anything, but your mail provider might apply spam filters before messages are downloaded in TB. That is why I suggest checking webmail.

Bitdefender might also have an anti-spam component. If you're unsure, try disabling or removing BD temporarily and apply the built-in Windows Security instead.