I'm not sure if this is the cause but the Server Name I use is outlook.office365.com and have had no problems. The Port should be 995 for POP too.

I can receive the messages but my older messages have disappeared. Since I have 15+ years of messages stored, I can't risk them. What happens to the filters I have created? I have 30+ filters currently !

what local folder? I thought the folder name on disk followed the server name.

Maybe but in my case the folders are Mail\localhost-1 Mail\localhost-2 & so on and Mail\pop3.live-2.com etc.

If I change the incoming POP sever name, the Local Directory is still being shown as the old one but all my old messages in Inbox have disappeared.

There is some sort of problem here. I do not know what it is. But you should not have server folders with localhost in the name as there is no mail hosted on localhost, unless you are using some sort of specialized software on your system such as is used by proton mail paid version

See https://en.wikipedia.org/wiki/Localhost

Maybe Mozilla changed something down the line but I have been using Thunderbird for more than 15 years & that's the way its setup. I didn't manually change anything. Since I am using POP, the mails are downloaded and stored on my local computer. So Thunderbird could have been referring that as localhost to mean that these are stored on my local computer (hence localhost). I am not using any other software other than Thunderbird as my email client.