I had the same problem - except that I didn't do any cleaning. I copied a folder of WinXP fonts into /usr/share/fonts. My mistake was copying a folder of Win8 fonts into /usr/share/fonts besides that. Some of the fonts were the same in folders - which made them not unique. The system might become confused. Besides, in my case Win8 fonts had permissions 100600, which made them unusable. After deleting Win8 folder Firefox started showing texts!
LM17.2 Rafaela 32 Mate, Core2Duo firstname.lastname@example.orgGHz, P5B-MX/WIFI-AP, 2GiB RAM, 1Tb SATA Ext4, 16Gib root, 2GiB swap, Logitech Mx-400 mouse, Bluetooth Level One MDU-0025USB