CSS grid is different on chrome and mozilla - ☔︎ ∙ I need Help!

The culprit here is your header. You can fix it by changing your .header to a grid-column of 1/6

Code

.header {
    grid-column: 1/6; 
                
    background-repeat: no-repeat;
    background-size: contain;
}

Your entire CSS should look like this now:

Code

:root {
    --header-image: url("https://strange-things-happen.neocities.org/sth%20daytime.png");
    --content: #483457;
}

@font-face {
    font-family: PixelOperator;
    src: url(fonts/PixelOperator.ttf);
}

* {
    font-family: PixelOperator;
}

.parent {
    margin: auto;
    max-width: 940px;
    height: 700px;
    display: grid;
    grid-template-columns: repeat(5, 1fr);
    grid-template-rows: repeat(5, 1fr);
    grid-column-gap: 0px;
    grid-row-gap: 0px;
}

body {
    margin: auto;
    background-color: white;
    color: #B71887;
    background-image: url("https://strange-things-happen.neocities.org/sth%20bg%20night.png");
    image-rendering: pixelated;
    background-attachment: fixed;
    background-position: 25%;
    background-size: cover;
    background-repeat: no-repeat;
}

* {
    box-sizing: border-box;
}

#container {
    margin: auto;
    max-width: 100%;
    max-height: 880px;
}

.header {
    grid-column: 1/6;
    background-repeat: no-repeat;
    background-size: contain;
}

.header img {
    height: 100%;
    width: 100%;
    object-fit: contain;
}

.two {
    display: grid;
    grid-column-start: 1;
    grid-column-end: 5;
    grid-row-start: 2;
    grid-row-end: 5;
    z-index: 0;
    pointer-events: none;
}


#newsdiv {
    position: fixed;
    height: 150px;
    width: 100px;
    overflow: scroll;
    z-index: 9;
    background-color: grey;
    color: black;
    padding: 1px;
    right: 380px;
    top: 80px;
    cursor: pointer;
    border: outset;
    text-align: center;
    text-decoration: none;
}

#newsdiv::-webkit-scrollbar {
    width: 5px;
}

#newsdivHeader {
    height: 18px;
    cursor: move;
    z-index: 10;
    background: rgb(6, 9, 58);
    background: linear-gradient(90deg, rgba(6, 9, 58, 1) 0%, rgba(98, 13, 116, 1) 58%);
    text-align: center;
    color: white;
}

/* navigation section!! */

/* this colors BOTH sidebars
if you want to style them separately,
create styles for #leftSidebar and #rightSidebar */
aside {
    display: grid;
    grid-column: 2;
    grid-row: 2;
    background-color: #FBDD6E;

    width: 150px;
    padding: -1px;
    margin: 5px;
    /* this makes the sidebar text slightly smaller */
}

.sidebarbtn:hover {
    filter: hue-rotate(230deg);
}

#navigationButton {
    filter: grayscale(80%);

}

.sidebarbtn {
    border: groove #F8B175;
    display: flex;
}


/* this is the color of the main content area,
between the sidebars! */

main {
    display: grid;
    grid-column: 3 / 5;
    grid-row: 2 / 2;
    background: bottom;
    background-color: #EEB3FF;
    background-image: url("https://strange-things-happen.neocities.org/dandelions.png");
    background-size: 100%;
    background-repeat: no-repeat;
    color: #B71887;
    flex: 1;
    padding: 20px;
    margin: 5px;
    border-image: url("https://64.media.tumblr.com/47d4d7688278aee63436613bc8843674/fb978ef4c9e61e33-3e/s500x750/4f5f3b69a346a35014643b171c933f2ba1acbcca.png");
    border: 2px dashed #B71887;
    order: 2;
}

#innerbox {
    width: auto;
    position: relative;
    background-color: #edeab7;
    padding: 5px;
    padding-right: 20px;
    margin: auto;
    image-rendering: initial;
}

#innerboxb {
    background-color: #edeab7;
    overflow: scroll;
    margin: auto;
    width: 650px;
    padding: 5px;
    image-rendering: initial;
}

.collumn {
    float: left;
    width: 50%;
    padding: 5px;
}

.center {
    display: block;
    margin-left: auto;
    margin-right: auto;
    width: 70%;
    text-align: center;
}

#musictv {
    display: grid;
    grid-column: 4;
    grid-row: 3;
    position: relative;
    z-index: 2;
    pointer-events: none;
}

#musicvideo {
    display: grid;
    grid-column: 4;
    grid-row: 3;
    z-index: 1;
    position: relative;
    align-items: center;
}

#chatbox {
    display: grid;
    grid-column: 2;
    grid-row: 3;
    width: 150px;
}

.gifsidebar {
    z-index: 2;
}

iframe {
    object-position: top;
}

/* what's this "order" stuff about??
allow me to explain!
if you're using both sidebars, the "order" value
tells the CSS the order in which to display them.
left sidebar is 1, content is 2, and right sidebar is 3! */

* #leftSidebar {
    order: 1;
}

#rightSidebar {
    order: 3;
}

.footer {
    background-color: #EEB3FF;
    display: grid;
    grid-column: 2 / 5;
    grid-row: 4;
    width: auto;
    height: 45px;
    padding: 5px;
    margin: 5px;
    border: 2px dashed #B71887;
    text-align: center;
    align-items: center;
    z-index: 2;
}

#spacestamp {
    -position: center;
}

h1,
h2,
h3 {
    color: #B71887;
}

h1 {
    font-size: 25px;
}

strong {
    /* this styles bold text */
    color: #882D19;
}

/* this is just a cool box, it's the darker colored one */
.box {
    background-color: #F7D394;
    padding: 10px;
}

/* CSS for extras */

/* BELOW THIS POINT IS MEDIA QUERY */

/* so you wanna change the width of your page? 
by default, the container width is 900px.
in order to keep things responsive, take your new height,
and then subtrack it by 100. use this new number as the 
"max-width" value below
*/

@media only screen and (max-width: 600px) {
    #flex {
        flex-wrap: wrap;
    }

    aside {
        width: 100%;
    }

    /* the order of the items is adjusted here for responsiveness!
since the sidebars would be too small on a mobile device.
feel free to play around with the order!
*/
    main {
        order: 1;
    }

    #leftSidebar {
        order: 2;
    }

    #rightSidebar {
        order: 3;
    }

    #navbar ul {
        flex-wrap: wrap;
    }
}