Firefox userChrome.css that makes it look a bit more like Safari on macOS

userChrome.css

.tabbrowser-tab .tab-close-button {
        order: -1 !important;
        padding-inline-start: 6px !important;
        margin-left: -4px !important;
        margin-right: -20px !important;
        z-index: 1 !important;
}

.tabbrowser-tab .tab-close-button:not(:hover) {
        color: rgba(0, 0, 0, 0) !important;
}

.tabbrowser-tab .tab-close-button:hover {
        background-color: color-mix(in srgb, currentColor 8%, var(--tab-selected-bgcolor)) !important;
}

/* Override some breaking defaults */
.tabbrowser-tab:not([labelendaligned], :hover) > .tab-stack > .tab-content > .tab-close-button {
        padding-inline-start: 6px !important;
        width: 24px !important;
}


.titlebar-buttonbox {
        position: fixed !important;
        top: 10px !important;
}

#nav-bar {
        order: -1 !important;
}       

#back-button {
        margin-left: 6.5em !important;
}       

.titlebar-spacer[type="pre-tabs"], .titlebar-spacer[type="post-tabs"] {
        width: 10px !important;
}       

To achive the look in the screenshot, the compact UI density has to be set:

user_pref("browser.compactmode.show", true);
user_pref("browser.uidensity", 1);