A problem that might appear when using access Facebook related to cookies :
Cookies are not enabled on your browser
This problem also happened to Google Chrome and Internet Explorer but I want to limit the troubleshooting with Firefox only.
How to Enable Cookies for Facebook on Firefox
Follow this steps
Check current Cookies setting by open Firefox
Click Tools -> Options
Click Privacy Tab
Click use custom settings for history
Accept cookies from sites : Accept third party cookies
Yes, as seen from the picture Firefox default setting is enable cookies.
To allow or block specific site just click “Exceptions” button.
Enter address of web site, for example I enter : facebook.com
and click Allow.
Status of facebook.com now allow. Every session from facebook automatically accepted.
Click Close and try to re create cookies problem. If you still face the problem you can close the browser and try again to reopen the website for sure.