change the authentication method in account settings to oAuth... all done.

Nothing like a load of misleading information like what the Yahoo/AOL boffins have come up with this time round.

No this did not change anything as authentication setting in account is already set to OAuth2. I have already tried and deleted account via Options/Security, where it appears as a yahoo account, and in Account settings. When I Add an account, after filling in the boxes with my email address, ident and password select continue and Thunderbird fills in boxes (including OAuth2). Then select Done and Yahoo asks me to log in to my account (which been set up previously as part of the security transfer). Having logged in to yahoo I get a message to allow Thunderbird to access my yahoo mail. Agree to this this and account appears set up, even get an email to say new sign in on Thunderbird, but no emails on Thunderbird. When I try and send email on aol account via Thunderbird get a message " An error occurred while sending mail. The mail server responded: Request failed; Mailbox unavailable. Please verify your mail address is correct in your account setting & try again." Sorry need more help

Authentication method options are: Normal Password Encrypted Password Kerberos/GSSSAPI NTLM TLS Certificate or OAuth2

oAuth is not an option nor can I type it in

I would assume you are joking, but you posted. oauth2 is the list entry to select.

No joke. When I try to add aol account Thunderbird automatically puts in OAuth2. Coincidentally TB has now started to ask me to sign in to my Blueyonder account with my password. Looks like this is not a problem you can help me with, its probably a problem with my machine so I will get someone to look at. Thanks for trying.