There isn't enough info to go one here and the ff Browser isn't a email program itself. So you need to look at the site email that your using to see what is happening.

I have firefox & my email provider found that the reason my emails are no longer displaying new to old is a server problem with FireFox. They display correctly on Microsoft edge.

Hi admacollins, the method of sorting varies from one email site to the next. Some display messages individually and you just need to click a column heading, others may group messages by priority, or into conversations, or in other ways, and you need to go through a menu or settings page to unravel that.

Do you want to mention what site you use? If it's Spectrum, there was another thread two days ago: https://support.mozilla.org/questions/1245360