To diagnose problems with Thunderbird, try one of the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

That doesn't solve it. I tried this on 2 different profiles. The problem is the new yahoo neo interface. I just moved from the Bat and the header pane shows correctly. Old messages show correctly in both clients. New messages do not in thunderbird, but when dropped into the bat, they show correctly THis all happened sometime last week The new format they are using is user@yahoo.co.uk [group]" <group@yahoogroups.com> Notice the order, spacing and bracketing. This is something for the developers to address. If anyone needs a screen shots so it can be reported properly, post back here

This is a Yahoo thing. Posts now come from the group address, with the actual sender's email address in the name field.

It looks like: From: "Joe Blow Joe@blow.net [Yahoo-Group-Name]"

<Yahoo-Group-Name@yahoogroups.com>

And "Joe Blow Joe@blow.net [Yahoo-Group-Name]" is treated as a name, rather than the address.

The actual sender address is correct in the X-Sender and X-Original-From fields, so an option to display either of of those fields in the columns rather than the Sender field for those folders would be very useful.

If you leave the display name for your Yahoo group blanc in your address book, Thunderbird will show the full e-mail adress, including the sender's

That doesn't work for me (OSX 10.9.3). The only way to see the sender address is to uncheck "Show only display name for people in my address book". And then I can see the munged From address I entered above. The correct address is available only in the X-Sender and X-Original-From fields.

For the developers; this is the what and why http://yahoogroups.tumblr.com/post/85163779041/dmarc-related-changes-in-yahoo-groups

Yes! Thanks a ton for this. So simple. Who knew? (well, you did.)

(replying to Hallvan's solution, above If you leave the display name for your Yahoo group blanc in your address book, Thunderbird will show the full e-mail adress, including the sender's)