We have seen cases where players are receiving an error saying FB Library Missing. It appears to be caused by an Add-on or browser Extension.
A quick way to test if this is the case is to start your browser in safe mode, incognito mode, etc where all Extensions/Add-ons are disabled just for that session.
Try the game you were attempting to play and see if you are no longer receiving the error.
If you are no longer receiving the error, that means it is definitely being caused by an add-on or extension in your browser.
Open up your browser and find the tools menu and look for Add-ons or Extensions. Once there disable all of the extensions, then you can start enabling one extension at a time until you trigger the one that is causing the error.
Here is a list of commonly found Extensions/Add-ons to cause this issue. This list is not all inclusive, but is just a list of reported Extensions/Add-ons that players have had that were causing this error.