Move or copy them to Local Folders.

Just how do you move 100s of email documents? One by one? That's too much

You can select more than one item.

http://xenos-email-notes.simplesite.com/417754237

Whilst you can drag and drop, I prefer to select then right-click and Move to...

Look at. Tools|Message Filters for a way to automate this.

I have looked long and hard. I see no option in Mozilla for one way sync. Can you check this please?

You choose to invent a name for this process that i have never heard before. So there is no button or setting labelled "one way synch".

The way to achieve what you're looking for is to make copies outside the Yahoo account, and Local Folders is the obvious place to do this in Thunderbird.

So you have two separate challenges.

1. Deal with the historic backlog of "hundreds of messages" . You simply have to move the messages. Select them in batches, move them to their new home. Be patient. http://xenos-email-notes.simplesite.com/417754237

2. From here on in, you can set up filters to automatically move all incoming messages in the Yahoo account to their new home.

And now your third challenge, having decided to store it all locally, is to backup that store against the inevitable and eventual failure of your own computer or hard disk. And to guard against failure of your backup device. And also to know that it is possible to restore from your backup.