see Cor-el's reply in "Are you aware of a possible bug in version 3.6.23? In the browser, for example, "two/" appears with the slash through the "o". Is this a known bug?"| Firefox Support Forum | Firefox Help

• https://support.mozilla.com/en-US/questions/892219

and again Cor-el -- Check zoom level -- Words are overlayed over other words in the text, how can I fix this problem. | Firefox Support Forum | Firefox Help

Try to disable hardware acceleration.

• Edit > Preferences > Advanced > General > Browsing: "Use hardware acceleration when available"

If disabling hardware acceleration works then check if there is an update available for your graphics display driver.