More likely you would use an addon or you would use Operating System related or Firewall software.

It would help to know Why are you trying to do this ?

If it is to prevent very young children accessing facebook it should be possible to get a suitable solution.

If it is in a workplace; or to prevent knowledgeable teens from accessing facebook; then mostly it is outside the scope of this forum, because you may then need to consider prevention of

• installing another copy of Firefox,
• using a separate OS from removable media.
• Or
  • you are looking for network wide solutions.
  • or at the least locking Firefox on some special installs

If you want, do a web search for child blocking software. Many of them are free.