Remove wrong placeholder-shown prefixing for Firefox

[timlg07]

Replacing :placeholder-shown with :-moz-placeholder breaks in Firefox. While placeholder-shown is working fine (supported since Jan. 2017), the :-moz-placeholder added by autoprefixer breaks selectors with :not:

Example:

input[placeholder]:not(:placeholder-shown) ~ label {
    top: 16px;
    font-size: 12px;
}

produces:

.form .field .input input[placeholder]:not(:-moz-placeholder)~label,
.form .field .input input[placeholder]:not(:placeholder-shown)~label{
    top: 16px;
    font-size: 12px;
}

This always applies, even if the placeholder is shown, because :not(:-moz-placeholder) evaluates to true.

I couldn't find any resources stating that :-moz-placeholder behaves like :placeholder-shown, so I wonder why it's in here?

[ai]

Have you just break all :-moz-placeholder?

I know about this problem, but we need careful solution.

I would like to accept PR, but I need from you to think about all edge cases.

[timlg07]

Do you know that it actually works in Firefox < 50? Because I couldn't find any proof for it. But even if it does, it should only be activated for Firefox < 50, as it's not needed in modern version and even breaks some selectors, like shown in my example🙈

[ai]

Do you know that it actually works in Firefox < 50?

It is mark as supported in Can I Use

https://caniuse.com/?search=placeholder-shown

it should only be activated for Firefox < 50

We are adding it because of KaiOS 2.5 (FF based browser for cheap phones very popular in Afrika). It is part of default browser set.

[timlg07]

That explains why adding 'not Firefox < 51' to the browserlist had no effect. I adjusted my browserlist and it works now. So I guess this stays and is intended? Thank you for the fast response!