Those characters are part of the Mathematical Alphanumeric Symbols Unicode block. Apparently the font that is used in your case lack those symbols or Firefox can't access this font.

• https://www.w3.org/TR/xml-entity-names/1D5.html

You may see text labels or little boxes with the hex code of the character if you do not have a font installed that covers this Unicode range or the font that is used doesn't cover this Unicode range or if you block downloadable fonts or if you do not allow pages to choose their own fonts.

@cor-el

Hi thank you for your reply. I think it has to do with Firefox because the same characters work fine on Safari. I have attached the image for your reference.