The screenshot shows some text in DejaVu Sans using my text code and Pango 1.14. "fl" and "fi" are being rendered as ligatures. I'm selecting inside "fl" to show how that works. Basically it's a hack --- a ligature with N characters is divided into N vertical strips of equal width which I pretend are "pseudocharacters", and then I use clipping to draw the individual pseudocharacters. This seems to work OK.
The screenshot also shows another feature of the new text code: our text layout objects (gfxTextRuns) can span DOM node boundaries. In this example the textrun spans the boundary of an <a> element. This enables us to form ligatures across those boundaries but thanks to partial ligature support, we can still style the link element correctly. Not just ligatures but also kerning and shaping benefit from cross-node text layout.
Note: When I said above that any substring should be selectable, that was a lie. Multiple Unicode characters can combine to form "clusters", for example a character with an accent, and selection should always be restricted to cluster boundaries. More consistent support for honouring cluster boundaries is another feature of the new text code. However, I currently don't support clusters spanning DOM node boundaries, because we really have no way to render those, not even a hack.
By the way, I noticed the hairline crack in the fi ligature. It's probably a rounding error that I need to track down.