I don't see why Firefox doesn't perform font substitution. It makes no sense to use the default font

I don't see why Firefox doesn't perform font substitution. It makes no sense to use the default font if it doesn't contain the necessary glyphs. Example:

Save fontawesome-webfont.woff

Convert it to TTF at everythingfonts.com

Install the converted TTF font.

Restart Firefox.

Under Options - Content - Advanced, uncheck "Allow pages to choose their own fonts".

Go to http://fontawesome.io/icon/chevron-left/

Instead of the actual chevron symbol, there are rectangles with the Unicode codepoint, even though that symbol is available in a local font. And yes, the converted TTF works fine; I tested on the same page with "Allow pages to choose their own fonts" checked and gfx.downloadable_fonts.enabled set to false.