Fixed element moving/jumping on Firefox Android

I have a fixed banner at the bottom of my website. Here is the styling for it

#mobile-cookie-policy {
    position: fixed;
    bottom: 0;
    width: 100%;
    height: 70px;
    text-align: center;
    background-color: #f7f7f7;
    border-top: 1px solid #ddd;
    z-index: 1;
    padding-bottom: 10px;
}

#mobile-cookie-policy p {
    font-size: 11px;
    text-transform: uppercase;
    letter-spacing: 0.5px;
    display: block;
}

#mobile-cookie-policy span {
    cursor: pointer;
    padding: 10px 20px;
    display: block;
}

And the element itself

<div id="mobile-cookie-policy">
    <span onclick="hideDiv('mobile-cookie-policy')" id="mobile-cookie-close">&#x2715;</span>
    <p>By using our website you are <br>agreeing to our <a href="#">use of cookies</a></p>
</div>

For some reason, on Firefox Android the fixed banner doesn't work it and jumps all over the place. Its default position is also slightly out of view. I can't work out why.

Here is a demonstration: https://imgur.com/a/FVsjt5k

EDIT: Curiously, I have discovered that it works on Firefox when the toolbar is set to be on the bottom, but not when it's on the top.

EDIT2: I have tried removing every other element on the page one by one, but it didn't help in any case.

EDIT3: I have also discovered that disabling "scroll to hide toolbar" makes it function as intended.

EDIT4: I am having the same issue with this simple website

<!DOCTYPE html>
<html lang="en" dir="ltr">
    <head>
        <meta name="viewport" content="width=device-width, initial-scale=1, minimum-scale=1">
        <meta charset="utf-8">
        <title></title>
        <style>
            #main {
                height: 3000px;
            }

            #fixed {
                background-color: red;
                bottom: 0;
                position: fixed;
                width: 100%;
                height: 70px;
            }
        </style>
    </head>
    <body>
        <div id="main">
        </div>
        <div id="fixed">
        </div>
    </body>
</html>