fi
Sometimes — just sometimes — the solution to a long standing problem is far easier than you initially thought. My quest for the ffi ligature, turned out to be one of those rare cases. I went as far as writing my own little markup language and adding an Entity Declaration to the XHTML Document Type Definition. Neither of these provided the result I was looking for. Silly me... I could have just used the fi ligature as a character in the sourcecode. The only thing that prevented me from doing so before was that my documents are ANSI encoded, even though the XML encoding says UTF-8. If I save a HTML document with UTF-8 encoding, it works perfectly fine, and it validates aswell.
...
D'oh!
P.S. I know this is a fi ligature, and not a ffi ligature... don't spoil the moment for me.