You can try removing the app from your Facebook account by going to Facebook > Settings > Apps > Angry Birds Friends > Remove app. Afterwards, you can relaunch it by clicking the link below.

https://apps.facebook.com/angrybirds/

Here are also other tips that may be useful:

• Clear the cache and cookies from your browser
• Check that your firewall or antivirus software is not blocking the game
• Check that you don't have "AdBlocker" or such plug-ins blocking the game
• Try playing the game with a different browser
• Check that you have the latest version of the browser
• Check that WebGL is not blocked on your browser

In case you are on Windows 10 device, you can also play the standalone version that is available in the Windows 10 Store - https://www.microsoft.com/en-us/p/angry-birds-friends/9nn6g50qnl1h