Had the same issue, like everyone else I,
1. Checked the MTU on my computer,
2. Disabled all the add-ons , started in safe mode and facebook opened once in safe mode.
3. Uninstalled / Reinstalled firefox > Same problem.
4. Uninstalled again through add/remove program and noticed that there were two versions of Adobe Flash Version 10. (I work in IT and this has been an issue on other machines, Uninstalled both Adobe Flash 10 ) and reinstalled firefox.
Was able to resolve my issue