> Example: <http://www.williamsauction.com/ca_all.php> -- There should be  
> up arrows and down arrows at the top of each column, but the Webdings  
> font isn't being used in Firefox, which shows "5" and "6". I've tried  
> specifying the font with an inline style in a span -- still doesn't work.

What Firefox/Win (and, incidently, Opera) is doing is correct. Your HTML  
markup has the characters '5' and '6'. They don't exist in the Webdings  
font, so the browser can't display them. It might be more correct to say  
that Webdings can't display ISO-8859-1 characters.

Anyway, the correct solution is to put the correct characters into the  
markup, namely &#9650; and &#9660;. I got those from the Unicode character  
charts <http://www.unicode.org/charts/>, specifically the "Geometric  
Shapes". Note that the hexadecimal Unicode codes need to be converted to  
decimal.

Webdings and similar fonts (eg Wingdings) really shouldn't be used at all.  
Always use Unicode for this sort of thing.