This is your first posting in this forum, so I do not know who told you anything in the past. But it did not come from this forum.

That it is asking for a password and knowing the simplicity of the telstra system. (It has to be simply because they don't hire anyone with any real skills to do anything anymore) I would take a wild guess you have an antivirus program that is blocking mail.

What exactly has occurred? What does the error message you see actually say. We know only what you tell us about your setup so right now we know nothing at all about the specific of your issue, only my bias from dealing with bigpod and telstra for 40 years as a customer (They were slow but the PMG did get it done right when they did do things). I still wonder how the call numbers can have exceed those expected for about the last thirty years and no one has been sacked for their failure to get it right just once. I have managed to reduce my dependence of the big Australian provider to only a mobile service. But when you live on the first left after the back of beyond your choices for things tend to be very limited.

It is possible your settings are not correct as Bigpond accounts apparently still have different server settings to general Telstra users. So about all I can do it point you to the web page with the settings on and suggest you check them against what you have in account settings in Thunderbird.

https://www.telstra.com.au/support/email/imap-pop-smtp-mail-server-settings

Pamela, did https://support.mozilla.org/en-US/questions/1509935#answer-1730175 help solve your issue? If so, please click "Solves the problem". If not, please update this topic.