This is like bug 442139, only it even happens at normal line-heights. Only happens with gfx.font_rendering.directwrite.enabled -> true. Glyphs are snapped to pixels vertically but default line-heights are not integer numbers of pixels.

See subpixel-lineheight-1a/b in attachment 488825 [details] [diff] [review] for a reftest.

Looks like something here has fixed this http://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=52c2f34cd686&tochange=b91e7cab0613 (The other possibility is that something in my patches for bug 605872, bug 600452, or bug 569770 was causing this to happen on tryserver, but I was observing the problem in nightly builds without those patches on a win7 system.) http://hg.mozilla.org/mozilla-central/rev/72cd1af86528

http://hg.mozilla.org/mozilla-central/rev/5ea89aae98b5 looks like it might have fixed this.