Firefox muestra fuentes ilegibles al imprimir

Para solucionar el problema(*) en Debian GNU/Linux, basta con instalar el paquete ttf-liberation.

apt-get install ttf-liberation

apt-cache show ttf-liberation
[...]
Description: Free fonts with the same metrics as Times, Arial and Courier
A set of serif, sans-serif and monospaced fonts from Red Hat with
exactly the same metrics as the (non-free) Microsoft Times, Arial
and Courier fonts, which implies those fonts can serve as a drop-in
replacement. The font family is named Liberation.
Homepage: https://fedorahosted.org/liberation-fonts/
[...]

(*) Claro, todo depende de las fuentes que esté utilizando la página que se visualiza.

Tracedump:

newBaseSize: 12pt
newBaseSizeInPt: 12