You can solve this issue just by simply logging into your account on webfonts.fonts.com and then just head to your projects tab and click on "Save project". Looks like the JavaScript included from there is cached and by saving the project again the JavaScript is updated with the latest bugfixes (like changing the embed font format on FF 3.6.13 from WOFF to SVG). I think this issue is related to the WOFF exploit on FF (tho I thought that was already fixed) which was discovered earlier this year.