Firefox doesn't have built-in email functions, so could you clarify what site, service, or program you're using?

• Webmail site/service such as Yahoo! mail, Hotmail/Outlook.com, or Gmail
• Business email
• Thunderbird or other desktop email program
• Phone/tablet email program

it is yahoo mail

On my Yahoo inbox, as I scroll down, more and more messages are fetched. Does yours just stop after 7 days?

If you check your folders, can you find the older messages there?

If not, you may need to submit the question to Yahoo support.