Firefox doesn;t do email.

I have used Mozilla Firefox for my email for about 5 or 6 years now.

Then you are using web mail. Email is downloaded to your PC by an email program or "client", whereas web mail is viewed on the "web" at a website - you only "download" what you want, like attachments. The actual web mails are stored at the web mail website or "portal".

You would have had to do some configuration of Outlook so it could retrieve your email from Yahoo, an error code from Yahoo wouldn't have created the necessary settings without input from you. The web mail services I have used over the years (none of which was Yahoo, so I can't help you with particulars at Yahoo) have had a setting to leave the email on the server when downloading a copy to the local email client (like Outlook). Look at your preferences or settings at Yahoo and change that setting. Try Yahoo Help for exactly which setting needs to be changed, or for support in general - your problem really isn't a Firefox support issue.