AFAIK this problem only applies when working with Wayland, but you can use this solution in Xorg as well if it occurs.

There is a close button in Firefox titlebar:

It looks kinda out of place, especially when using a standalone Wayland compositor such as Niri or Hyprland.

You can remove this button and spacing to the left of it with the following CSS. Add it to your userChrome.css or create it you haven't already. Refer to this guide for more information.

/* Remove close button at the top right corner. */
.titlebar-buttonbox-container {
    display: none;
}

/* Remove close button spacing. */
.titlebar-spacer {
    display: none;
}