I was having a similar problem using Ubuntu 12.10. I did not realized that the problem started after I tried some dual screen configurations. When I went back to use just the Laptop Screen the resolution of the screen was set to 1360x786 pixels (there is such option) but my monitor has 1366x786 pixels. This 6 pixels difference has a minor impact on big fonts or pictures and etc... but the fonts in the browser are affected. Just set the resolution to match my screen native again and solved the problem.