Could you please help me?

I've tested email with text with applied font: 'Modern, sans-serif' (also tried 'Modern, Arial, sans-serif').

In all web-based clients, in Firefox version, this text doesn't render. Just empty space.

I've tested this text locally on 2 machines.

  1. First has installed Modern font. And here I can see text with applied Modern font.
  2. Second hasn't installed Modern font. Here I see text with Arial applied font (when I tried with Arial fallbacks).

In litmus I always see only empty space instead of text.

Is there some issues with litmus or with my email content?