I recommend these two quick steps for a friendlier, more consistent, and great looking Gnome desktop:
Step 1. One Font throughout Gnome
In System > Preferences > Appearance…
Note that all fonts are the same. Use any font you like, but I recommend you try FreeSans.
The exact numbers aren’t so important, just that they result in big letters on your screen. Because big letters on a big LCD with subpixel smoothing looks great.
Step 2. One Font on the Web
In Firefox’s Edit > Preferences > Content > Fonts & Colors > Advanced…
Note the minimum font size (big) and that the “Allow pages to choose their own fonts” option is off.