Open up Firefox and type 'about:config' into the urlbar. Accept the warning it gives you, then type 'layout.css.devPixelsPerPx' into the config page's search bo

Open up Firefox and type 'about:config' into the urlbar. Accept the warning it gives you, then type 'layout.css.devPixelsPerPx' into the config page's search box. Double click on the devPixelsPerPx preference, and type in a value such as 1.5 or 2, then click OK. Now the whole browser will be zoomed by that factor.