.env-list-panel {
    $width: 275px;
    box-sizing: border-box;
    width: $width;
    margin-left: -20px;

    &:before {
        @include diamond($white);
        top: -8px;
        left: 30px;
        width: 13px;
        height: 13px;
        border-width: 1px 0 0 1px;
        border-style: solid;
        border-color: rgba(0, 0, 0, 0.1);
    }

    .env-list {
        margin-left: 0;

        &__input {
            box-sizing: border-box;
            padding: 10px 10px 8px 10px;
            margin: 10px 10px 0 10px;
            height: 100%;
            width: $width - 20px;
        }

        &__environment {
            position: relative;
            // The items need to be above the diamond.
            z-index: 1;
            list-style-type: none;
            padding: 10px 20px;
            margin-bottom: 0;
        }

        &__environment:hover,
        &__environment:focus {
            background-color: $hover-background;
            cursor: pointer;
        }

        &__message {
            padding: 10px 10px 0 10px;
        }
    }
}

.env-list-panel {
    $width: 275px;
    box-sizing: border-box;
    width: $width;
    margin-left: -20px;

    &:before {
        @include diamond($white);
        top: -8px;
        left: 30px;
        width: 13px;
        height: 13px;
        border-width: 1px 0 0 1px;
        border-style: solid;
        border-color: rgba(0, 0, 0, 0.1);
    }

    .env-list {
        margin-left: 0;

        &__input {
            box-sizing: border-box;
            padding: 10px 10px 8px 10px;
            margin: 10px 10px 0 10px;
            width: $width - 20px;
        }

        &__environment {
            position: relative;
            // The items need to be above the diamond.
            z-index: 1;
            list-style-type: none;
            padding: 10px 20px;
            margin-bottom: 0;
        }

        &__environment:hover,
        &__environment:focus {
            background-color: $hover-background;
            cursor: pointer;
        }

        &__message {
            padding: 10px 10px 0 10px;
        }
    }
}

Fix env switcher layout in Firefox.
Fix env switcher layout in Firefox.