Private Sync

http://avih.github.io/blog/2013/05/31/private-firefox-sync-server-in-5-mins/

Custom style sheet per domain

In the firefox profile directory (%APPDATA%\Mozilla\Firefox\Profiles\[USERNAME-PROFILE]\chrome\userContent.css)
Copy-and-paste and change the [DOMAIN.tld] one of this stylesheets to your userContent.css file
Restart of firefox is required.

Domain prefix

@-moz-document domain(www.belastingdienst.nl) {
body {background-color:#000066 !important;}
}

URL prefix

@-moz-document url-prefix(http://www.belastingdienst.nl/) {
  body {background-color:#000066 !important;}
}

Custom background picture

@-moz-document domain(www.belastingdienst.nl) {
body {background:#fff url(http://[DOMAIN.tld]/bd-bg-body-2.png) 0px 0px repeat-y !important;}
}

The domain ‘www.belastingdienst.nl’ will change the background color to blue 🙂