The unthinkable happened. You can download and send emails from most popular free email accounts as if you paid up. Who doesn't have a few free email accounts? One for posting on Craigslist, one for registering for some free services, one personal, one for spams, etc. It's a pain to switch users in Yahoo, who punishes you by forcing you to read more advertisements. One account with multiple ID's is not safe. One day you will review the wrong ID to the wrong people.
Thunderbird has an extension called WebMail. After installing that and restart, you can install modules specific to each email service, including Yahoo, Gmail, Hotmail and many more.
Honestly this one escaped me for a while. For some reasons, it doesn't appear on the thunderbird recommended extension list, not on the popular list, nor you find it via casual search. Maybe the reason is just that the web pages are outdated. Perhaps the whole thing is understandably rather buggy in the first years. More likely, the email service providers can screw you up at any time to revenge their lost of business. Now after so many revisions of the software, I don't think they are going to do anything to sink it.
The installation is as easy as setting POP or SMTP accounts, as described in the instructions, though a little outdated. I would suggest first to setup the SMTP accounts first if possible, even if you don't intend to send emails via Thunderbird. Installation of POP accounts is as described, server is always localhost, and user is always user@domain - your full email address. Other details needed are display IDs that you can check later. What is missing from the instructions is that you need to go to add-ons, click on the email service module, eg Yahoo, and check the options. For Yahoo the default mode is the old version, most people would have switched to the new beta version, and that is the option that you have to set. It took me a while to get it working because of this.
In theory, Yahoo can screw you up anytime by changing their own protocol, eg from old version to beta. It's safe if you run Yahoo messenger and login to your most spammed accounts. If on a day that you don't seem to have new email notifications, there must be something wrong.
Webmail warned against setting up multiple SMTP accounts to send email. It didn't say whether it doesn't work at all or there will be some mixed up sometimes. I wouldn't do that anyway. I prefer login first to Yahoo to make sure that the account is the right one, before sending any email.