This is a known issue with Firefox and is not specific for the ReadCube Web Reader. Please see:
It is possible to manually enable DirectWrite in Firefox. We provide the directions below but please note, this does not guarantee optimal font display and may affect browser speed. To manually enable DirectWrite font rendering in Firefox:
1) In the address bar, type “about:config” and press Enter
2) Search for the preference name: “gfx.font_rendering.directwrite.enabled”
3) Double-click on it, and set the value to “true”
4) Restart Firefox and check the web reader
5) If the problem persists, please repeat the above for: "gfx.font_rendering.directwrite.force-enabled"
6) Double-click on it, and also set this value to "true"
7) Restart Firefox again and check the web reader
If the above steps do not address the issue, unfortunately the only remaining advice is to please try another web browser (such as Chrome, Internet Explorer, Safari or Opera). You can also reach our customer support at firstname.lastname@example.org. Since this is a general known issue with Firefox, please direct any specific requests for information about the browser’s hardware acceleration/DirectWrite settings to Mozilla Firefox support.