Inspired by this question on Meta: Is the mark to the left of the badges supposed to be a square or a checkmark?
On my FireFox, the "check" mark appears as a box with numbers in it, rather than a tick:
On Meta, they suggested checking your font settings, but I haven't seen anything that looks obviously untoward:
(I recently changed the encoding to UTF-8, as suggested, but it doesn't seem to have made any difference.)
The other recommendation proffered was to check out the List of Unicode Characters and use it to identify what's missing. The tick is in the "Dingbats" section - as far as I can tell, the only one of that section that my machine is rendering is the aeroplane (2708).
Does anyone know what I can do to fix it?