If you answered yes, you have just found the solution. Keep on reading.
If you are new to styling placeholder text or perhaps have just not noticed, Firefox in version 19 and beyond by default applies an opacity of 40% (0.4) to placeholder text, as per the documentation.
While I have known about this particular caveat for a while, it turns out a lot of developers who I have asked were not aware that this happens. So next time you change the colour of placeholder text in Firefox, set the opacity to 1 and you should be fine.
See below for an example of placeholder text being set white and the opacity fix being applied for Firefox.
input::-webkit-input-placeholder,
input:-moz-placeholder,
input::-moz-placeholder,
input:-ms-input-placeholder {
color #FFF;
}
input:-moz-placeholder,
input::-moz-placeholder {
opacity: 1;
}