Got a call a few minutes ago from Lua, who had just called a few hours ago inviting me to a party. She wanted to print some things out for the guest of honour (who is Japanese), but she was just getting question marks for the Japanese text.

I don't know if there's a built-in Japanese font, but the Bitstream Cyberbit font seems reasonable (not that I know what the characters are supposed to look like, of course). Just download, extract, right-click on the TTF and tell it to install. Once you've done that, restart Firefox, go to Preferences | General | Fonts and Colours, choose Japanese in the "Fonts for" drop-down, then select Bitstream Cyberbit in each of the Serif, Sans Serif and Monospace font drop-down boxes.