The font settings you chose in gnome preferences, "subpixel smoothing" (subpixel + full hinting), aren't used by firefox which is apparently using "best shapes" (grayscale + medium hinting)
Gnome exports Xft resources (see output of: xrdb -q |grep Xft)
You should see rgba=rgb (subpixel), and hintstyle=hintfull (full hinting)
Now I'm not sure why firefox doesn't use that. Maybe you've conflicting fontconfig settings. Do you have a ~/.fonts.conf file?
The font settings you chose in gnome preferences, "subpixel smoothing" (subpixel + full hinting), aren't used by firefox which is apparently using "best shapes" (grayscale + medium hinting)
Gnome exports Xft resources (see output of: xrdb -q |grep Xft)
You should see rgba=rgb (subpixel), and hintstyle=hintfull (full hinting)
Now I'm not sure why firefox doesn't use that. Maybe you've conflicting fontconfig settings. Do you have a ~/.fonts.conf file?