Firefox browser isn't a email program. For that issue you need to check your email provider setups for that settings.

Hi Geo, are you using a popular site like Gmail, Outlook, or Yahoo to view your mail? The settings vary between sites, so if you can't find the settings, we'll need more info to help you get there.

My guess it may be about Yahoo as they recently changed how Yahoo mail looks now much like they did back in December with the Yahoo site.