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:
Usa tu <a href="https://support.mozilla.org/kb/difference-between-imap-and-pop3">contos POP</a>? Diversemente del contos IMAP, le sol copia de tu posta pop es in tu profilo local de Thunderbird. Si tu desira adder un conto POP al version beta, tu optiones es: