Favicons on the left with hover for close button in Firefox 57

userChrome.css

/*
 * Adapted from https://gist.github.com/bramus/724503
 * with help from https://support.mozilla.org/en-US/questions/1157451
 *
 * Do not remove the @namespace line -- it's required for correct functioning
 * set default namespace to XUL
 */
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* move favicon, throbber and text to the right so that the close button appears on the left */
.tabbrowser-tab .tab-throbber,
.tabbrowser-tab .tab-icon-image,
.tabbrowser-tab .tab-close-button {
    -moz-box-ordinal-group: 0 !important;
}

/* hide close button initially */
.tabbrowser-tab .tab-close-button {
    display: none !important;
}

/* show close + hide throbber & favicon on hover */
.tabbrowser-tab:hover .tab-close-button {
    display: block !important;
    margin-right: 3px !important;
    margin-left: -1px !important;
}

.tabbrowser-tab:hover .tab-icon-image {
    display: none !important;
}

.tabbrowser-tab:hover .tab-throbber {
    display: none !important;
}

/* app tabs should not behave like regular tabs, so let them stay normal (viz. don't show close on hover) */
.tabbrowser-tab[pinned]:hover .tab-close-button {
    display: none !important;
}

.tabbrowser-tab[pinned]:hover .tab-icon-image {
    display: block !important;
}

/* disable top tabs if using a vertical tabs extension */
/* #TabsToolbar { visibility: collapse !important; } */

rev. 2 fixes tab labels shifting on hover in 57.0b2

rev. 3 adds a modification, disabled by default, to turn the top tab bar off in the case of a vertical tab extension

Thanks for this! I wish this was the standard behaviour on MacOS.