Have you witnessed any problems with the plugin causing text to be invisible when a page loads in firefox? I've had that happen twice now. Both times, I could tell the text was there (and it was highlightable) but it was invisible. I hit F12 to open firebug and then text magically became visible!
I have w3 total cache on as well. I'm going to disable that and see if I can take steps to narrow down the problem. But my gut is that the problem has to do with the combined CSS somehow not getting interpreted right by Firefox. I just don't understand why it's hit-and-miss.