graeme_p - 7:48 am on Sep 8, 2013 (gmt 0)
Sorry, I thought you were dealing with a SITE problem rather than an FF problem and meant to say its a problem with your FF install or settings.
The optimum layout.css.devPixelsPerPx should be system dpi divided by 96 (divisor may be different on windows):
The default is -1. Auto?