This is a Unicode thing. If the font Firefox is using in the subject line doesn't have the Unicode character referenced, it will display a box instead with the glyph's ID in it; in this case (2728). Either change the font that you're using in Firefox to a more Unicode-friendly font, or try installing one of the free Unicode fonts available. In certain fonts the character is a symbol called "Sparkles" and looks like this:
It is found in symbol fonts such as Dingbats. Whoever is sending you these emails thinks you are using a Unicode compatible font, or doesn't care. Or they've got a Mac. Or you have.