[Bug 19524] Re: Firefox does not pay attention to GNOME DPI setting

[Xavier]

layout.css.dpi=-1 -> 96 dpi minimum
So you didn't need to change that.
But the dpi setting only affects the fonts sized in points, not the ones in pixel. That must be the difference between the websites you are using, and there is nothing you can do about that.
(that's also the reason why some ppl always recommend 96 dpi for web browsing)

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Firefox does not pay attention to GNOME DPI setting
https://launchpad.net/bugs/19524