What Linux are you running? "fc20" I guess that's Fedora? So it looks bad on Firefox on Fedora but fine on Ubuntu. That's a Fedora issue not a Firefox or general Linux issue.
Yes, fc20 stands for Fedora 20. The difference between it and Ubuntu isn't that big if I zoom in: http://imgur.com/ptlQEmf
Based on the other comments I got, the small differences could be caused by different anti-aliasing settings or font engines (I'm currently using freetype-freeworld-2.5.0.1-5 for example).
http://imgur.com/6nKzwZq
What Linux are you running? "fc20" I guess that's Fedora? So it looks bad on Firefox on Fedora but fine on Ubuntu. That's a Fedora issue not a Firefox or general Linux issue.