OpenType feature control 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.

Font: Bold Italic
Small caps: Off
Synthetic (font-variant: small-caps)
OpenType (smcp)
Numeral style: Default
Lining (lnum)
Oldstyle (onum)
Numeral spacing: Default
Proportional (pnum)
Tabular (tnum)
Fractions: Off
Normal (frac)
Alternate (afrc)
Slashed zero: Enabled (zero)
OpenType kerning: Enabled (kern)
Ligatures: Common (liga)
Discretionary (dlig)
Historical (hlig)
Swash forms: Enabled (swsh)
Stylistic sets: ss01
ss02
ss03
ss04
ss05
CJK character forms: Default
Simplified (smpl)
Traditional (trad)
Feature string:
The Quick Brown Firefox acts swiftly