Firefox does not display Japanese characters on buttons in several websites. There are no boxes--such as there would be with a font issue or an encoding problem. The text simply does not show up at all. Changing the font in Preferences > Content > Advanced to a Japanese font does not resolve the issue. Disabling various extensions does not resolve the issue.

This is affecting several websites when I set the language or locale to Japanese, but the most obvious example is in Gmail's compose message page, when the display language is set to Japanese. The text on the Send and Discard buttons are missing (There is no visible text.) The Report Spam button has incorrect text. (It says "み").