Hey! I have the same problem on two blogs with 6.13. On a test site which is a fresh WP 3.5.1 and my real blog as well. I did solve it by changing the ftp permissions to 755 which I discovered by the F12 command but I don’t know if it’s practical or something you could fix to prevent when installing it as it was a headache for me to figure it out.
I did the F12 console like you wrote and here is my error code when I load up a new post, change the font, click save, then update.
Resource interpreted as Stylesheet but transferred with MIME type text/html: “https://adored.davidshotme.com/wp-content/plugins/font/generateEditorCSS.php”. wp-tinymce.php:2
GET https://adored.davidshotme.com/wp-content/plugins/font/font_files/vinterstad.woff 403 (Forbidden) /wp-content/plugins/font/font_files/vinterstad.woff:1
GET https://adored.davidshotme.com/wp-content/plugins/font/font_files/vinterstad.ttf 403 (Forbidden) /wp-content/plugins/font/font_files/vinterstad.ttf:1
