You can choose any font you have locally available by setting the "reader.font_type" to the font name in about:config

You can also set "reader.content_width" to values beyond the 9 allowed by the UI to have the text take up even more of your screen width. Setting it to 12 is just about perfect for me.

These values will be lost if you update the font style or the width via the Reader "Text and Layout" menu though. For fonts, you might be able to avoid that by putting fonts you want to be able to select from in the "reader.font_type.values" list but I haven't tried that.