Firefox not rendering Unicode private use area characters

[Ian Pilcher]

I'm working on documenting some Python code with Sphinx and the Read the
Docs theme.  This theme uses a couple of Unicode "private use area"
characters - U+F015 (house) and U+F06a (exclamation mark in circle).

These characters are not rendering when I use Firefox to view my locally
generated documentation (although they *are* rendered when I view RTD
documentation online).  These characters render just fine in Google
Chrome, LibreOffice Writer, and KCharSelect, so I don't believe that I'm
missing a font.

Anyone have any idea what might be going on?