Remove empty space before tabs in Firefox compact theme

userChrome.css

/*
 * Do not remove the @namespace line -- it's required for correct functioning
 * Put this file under ~/Library/Application Support/Firefox/Profiles/[you]/chrome/
 */
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */
/*
 * FF < 57
 */
#TabsToolbar {
	padding-left: 0 !important;
}
/*
 * FF >= 57
 */
#TabsToolbar .titlebar-placeholder:first-child{
	width: 0 !important;
}

Firefox 113

Now it's really quite easy

userChrome.css

.titlebar-spacer {
	display: none !important;
}

For macOS, I recommend the following CSS as you want a little padding between the buttons and the tabs.

.titlebar-spacer {
        width: 12px !important;
}

This way they are aligned perfectly with the refresh button as well.