I have been working on adding OpenType font feature control to Gecko. This is currently available in Mac OS X and Windows nightly builds of Firefox. For Windows, you will need to change the gfx.font_rendering.harfbuzz.level preference to 1 in about:config in order to access the OpenType features.
Several pages are available here that show examples of how CSS control of font features might be used to enhance web pages:
There is also an OpenType Feature Playground page where you can experiment further with features, using whatever OpenType fonts you may have available.