The problem seems to be a css one.
.wpcf7-form p {
color: #0000;
font-size: 12px;
margin-bottom: 0;
margin-top: 0;
padding-bottom: 10px;
padding-left: 0;
padding-right: 0;
padding-top: 0;
width: 290px;
}
changing color: #0000; into color: #000; or color: #000000; shows the text again.
color: #0000; renders the text transparent in FF.
Quote:
Firefox 49 has added the support for 4-digit (#RGBA) and 8-digit (#RRGGBBAA) hexadecimal notations for RGBa colour values. Due to this change, previously invalid values could be valid, yielding unexpected results. A typical example is #0000, which is now parsed as transparent. You may want to double check your stylesheets for such errors.