How to style placeholder text?

The following partial code represents a Bootstrap 4 form. I am trying to style the placeholder text, but haven't been able to select it.  Using FireFox browser, I've tried variations  of the following: Hopefully anybody show me how this is done.

CSS

.l-footer {
  display: flex;
  position: relative;
  flex-flow: column wrap;
  justify-content: flex-start;
  align-items: center;
  background-color: $black-black;

  &-left-section {
    flex-basis: 1 1 0;
    display: block;
    order: 3;
    text-align: center;
    border: .1rem solid $white-white;
    padding: 2rem 3rem;
    margin: 0 auto;
    max-width: 90vw;
  
    &__form-container {
      display:block;
      min-width: 100%;
      margin-bottom: 3rem;

       &form.newsletter-signup::-moz-placeholder {   /* Firefox 19+   For newer FF also tried  &form.newsletter-signup::placeholder  */
        @media (max-width: 319.98px) {
          font-size: $font-size-base * .65;
          color:green;
        }

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HTML

<div class="l-footer container-bleed">
        <div class="l-footer-left-section container">
          <p class="l-footer-left-section__newsletter">
            SUBSCRIBE TO MY<br>QUARTERLY<br>NEWSLETTER
          </p>

          <div class="l-footer-left-section__form-container">
            <form class="newsletter-signup" action="http://www.stevehonnenrealtor.com/sendForm.php" name="form-signup-newsletter">
              <input type="hidden" name="env_report" value="REMOTE_HOST,REMOTE_ADDR,HTTP_USER_AGENT,AUTH_TYPE,REMOTE_USER">
              
              <div class="form-group">
                <label class="sr-only" for="input-email">Email</label>
                <input type="email" class="form-control" id="input-email" placeholder="Enter Your Email">
              </div>

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