Hi Coyla, is this a problem in Firefox when using mail.yahoo.com, or a problem in Thunderbird? Want to make sure your question is in the correct category.

I use Thunderbird 38.1.0. Also, I should have been clear on the fact that it is my [email address removed] (they use yahoo mail) address that won't download the mail. I have been using Thunderbird for over a year now, and this is the first time I have had a problem. It just started yesterday.

Email address removed by moderator. You are posting in a public forum. Please do not post sensitive information as outlined in the Mozilla Support rules and guidelines

To diagnose problems with Thunderbird, try 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.