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:
Wužywaśo <a href="https://support.mozilla.org/kb/difference-between-imap-and-pop3">POP-konta</a>? Na rozdźěl wót IMAP-kontow je z nimi jadnučka kopija wašeje POP-mejlki we wašom lokalnem profilu Thunderbird. Jolic cośo betawersiji POP-konto pśidaś, móžośo: