Are you using <a href="https://support.mozilla.org/kb/difference-between-imap-and-pop3">POP accounts</a>? Unlike IMAP accounts, the only copy of your pop mail is in your local Thunderbird profile. If you want to add a POP account to beta, your options are:
Wužiwaće <a href="https://support.mozilla.org/kb/difference-between-imap-and-pop3">POP-konta</a>? Na rozdźěl wot IMAP-kontow je z nimi jenička kopija wašeje POP-mejlki we wašim lokalnym profilu Thunderbird. Jeli chceće betawersiji POP-konto přidać, móžeće: