I noticed the same CSS link in the <head> of pages on a local non-internet facing Apache site at home. Looking further I found it in just about any page I browse with Firefox. Like @dude mentioned, I see it when I have the DuckDuckGo Privacy Essentials extension activated.
I found a reddit post that seems to shed some light https://www.reddit.com/r/duckduckgo/comments/14r3vde/does_duckduckgo_privacy_essentials_firefox_uses/