Change in addressee format
Latest update has changed the addressee format whereby all recipients are now either on the TO: line or BCC: or CC: lines as entered by the sender. Before you could enter a number of recipients on lines that stacked up vertically as recipients were added. Then by using a pull down in the left margin of each line change the recipient status to CC: or BCC:. Changing all recipients to BCC would provide for email address masking of the recipients. Now a recipient is required on the TO: line thereby unmasking his address. Putting the sender's address on the TO: line and then placing all recipients on the BCC: line is a workaround, but not as obvious a solution as was the earlier format. So why change something that wasn't broke?
Opaite Mbohovái (1)
Because the new functionality is much more powerful https://support.mozilla.org/en-US/kb/addressing-email#w_changes-to-the-addressing-area-in-thunderbird-78 - addresses can be manipulated in multiple ways, drag and drop for example.