Placeholder text is same color as actual text in Firefox

[helenyhou]

I don't love targeting internal browser CSS, but in the case of placeholder text in Firefox, I think we need to. For example, in the media modal, "Describe this image..." is used as a placeholder but rather appears as though it's the user's input and is a bit confusing, especially because it doesn't disappear on focus.

Done both for the admin at large and for the media modal itself in the attached patch.

In 22986:

In Firefox, style placeholder text a dark gray since the browser doesn't do it for us. props helenyhou, fixes #22689.

Note that Firefox will ​switch to `::-moz-placeholder` in version 19. That is not far away, I think we should use both selectors now for a while.

Are we going to remember to remove one/both later, then? This is why I hate targeting this mess. We have :-moz-placeholder in a couple other places as well (color picker and toolbar). Maybe worth a separate ticket to address them all instead.

Replying to helenyhou:

Are we going to remember to remove one/both later, then?

I am updating ​this beast regularly, so I can promise not to forget it. :)

A separate ticket for all occurrences would probably be better, yes.

According to MDN, they are "deprecating" it. I don't see it saying it will be "removed". Let's revisit in 3.6?

A separate ticket for all occurrences of :-moz-placeholder sounds good.