Firefox menus and dialogs missing text

All Firefox (38.0.1-r1) menus and dialogs (including popups and form fields) are missing text as shown below.

Strangely, the web pages themselves have text, but the user interface elements do not.

I have installed ttf-freefont, font-bitstream-100dpi, ttf-dejavu, ttf-liberation, and many more.

Other gtk applications (xfce4-terminal, pcmanfm, thunar) do not appear to have this problem.

(from redmine: issue id 4248, created on 2015-05-25, closed on 2019-06-19)

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