Is the problem there in safe mode (Help/Restart with Add-ons Disabled)? Can you add recipients from the Contacts sidebar (F9 in a Write window)?

Problem is there in safe mode. BTW, problem I am referring to is that additional lines are not available to add new recipients (with the ability to specify each additional recipient as primary, cc, or bcc). I do notice now that I am able to add new primary recipients on the same line as the original recipient. But this is NOT a solution to the problem by any means because I am NOT able at all to add cc or bcc recipients. However, yes, I AM able to add new recipients, primary, cc, and bcc using the Contacts sidebar (F9 in the write window) --- and this is good to know --- with this sidebar I do have a way to add any new kind of recipient I need to. Having said that, however, I sure would like it to work the way it always has for me, namely that there are always new recipient lines available to add new recipients and i just click on the short menu on the left to specify which kind of new recipient I am adding. In fact, come to think of it, the Contacts sidebar seems to allow me to add the cc and bcc recipients only if they're in the address book and I might need to add some that are not in the address book!

Ah - Tbird changed the way that recipients are added - now they are all in one row instead of one row per recipient. Are you still unable to type new email addreses on the same To line? Do not try adding any punctuation, just type and hit enter. This article explains that and also how to add CC and BCC. https://support.mozilla.org/en-US/kb/addressing-email#w_changes-to-the-addressing-area-in-thunderbird-78

Oh my gosh! I didn't realize (obviously!) that this is a change in TB 78! Sorry to have reported this as a "problem." I'm not sure what the best way is to get educated on changes in a new release. I guess from now on I'll just have to be sure to read the release notes more conscientiously and carefully! Thanks to everyone who jumped in here to help me and educate me!