I did not mean to set a default homepage, I meant to set a new branding logo in the default firefox homepage. (To change the default firefox logo on the homepage to a custom one).

That would be this code in userContent.css.

@-moz-document url-prefix(about:home), url-prefix(about:newtab) {
 .search-wrapper .logo-and-wordmark .logo {
  background: url("about-logo.png") no-repeat center !important;
 }
}

In Firefox 69 and later you need to set this pref to true on the about:config page to enable userChrome.css and userContent.css in the chrome folder.

• about:config => toolkit.legacyUserProfileCustomizations.stylesheets = true
• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

More info about userContent.css and userChrome.css in case you are not familiar: *https://www.userchrome.org/what-is-userchrome-css.html

• https://www.userchrome.org/how-create-userchrome-css.html
• https://www.userchrome.org/firefox-changes-userchrome-css.html

It worked, but is there also a way to change the favicon of the default homepage?

Do you mean the favicon on the Tab bar or otherwise ?

yeah exactly that