I have for your pleasure the setup I use with Thunderbird + Yahoo Mail (Free).
First you need to activate pop mail on Yahoo Mail
. Login and press Settings (Sorry, I have it on Norwegian, so I don't know how it will appear if you have another language). From there click on Pop-access and forwarding, then just activate the check box next to "Web and POP access". Now your POP account should be activated.
Then open Thunderbird and start the Account wizard (it will start automagically if you don't have an account set up).
The first you will see is something like this:
You want an Email account (DUH!). Next:
This is just the name and the email you want shown to other when you send them. Generally you want your email and your name there. Next:
This is where you set up incoming server, if you activate the check box it means it will delete the emails online after downloading them, so you just have your local copy (I think). Next:
This is your login name for yahoo. Next:
Just a friendly name for your account, it doesn't matter what you call it.. Next:
Look over your settings and make sure they are correct, you might want to go back and correct them if anything is wrong. Since I already have an outgoing server set up the wizard did not ask me for setting that up to, be fear not, here are my settings:
And there you have it, that should work perfectly, I hope..