Set a folder named chrome on C:\Users\username\AppData\Roaming\Mozilla\Firefox\Profiles\xxxxxxx.default-release. Create userChrome.css and userContent.css on chrome folder.
To improve performance, Firefox 69 and later will disable loading of userChrome.css and userContent.css by default. Your CSS customizations are not going away; all you will need to do is set the pref toolkit.legacyUserProfileCustomizations.stylesheets to true in about:config.
/*remove new tab favicon*/
.tabbrowser-tab[label="New Tab"] .tab-content .tab-icon-image {
display: none !important;
}
.tabbrowser-tab[label="Private Browsing"] .tab-content .tab-icon-image {
display: none !important;
}
/*replace about logo*/
@-moz-document url("chrome://browser/content/aboutDialog.xhtml") {
/* change logo png, svg, even gif anims */
#leftBox {
background-image: url("logo.png") !important;
background-position: left !important;
background-repeat: no-repeat !important;
}
}
/*replace favicon*/
.tab-icon-image[src="chrome://branding/content/icon32.png"] {
content: url("foxlogofavicon.svg") !important;
} /*replace new tab logo*/
/*home*/
@-moz-document url(about:home),
url(about:newtab) {
.logo {
background-image: url("logo.png") !important;
}
body {
background: #4d5d53 !important;
--newtab-background-color-secondary: #3f4c44 !important;
--newtab-primary-action-background: transparent !important;
}
}
/*private home*/
@-moz-document url(about:privatebrowsing) {
.logo {
background-image: url("logo.png") !important;
}
}