I answered a similar issue on Firefox Connection with Facebook this morning with this link: https://community.king.com/en/candy-crush-saga/discussion/comment/378331#Comment_378331 Firefox seems to be problematic these days. To make this problem go away, would you be willing to uninstall Firefox and switch to Google Chrome instead? Just go to Google, download and install the App. Make sure that it is fully updated and you're good to go. I've used Chrome for years and never had a single problem at all. If I were in your situation and my browser is very uncooperative, I would easily switch over to a problem free one. Besides, they are free to use.
So I guess then we have to allow Facebook to track the websites that we visit and then see those stupid pictures/ads! I guess we have to weigh them - not able to play vs getting those stupid ads! Thanks for letting me know @Pinguu and I am going to save this picture just in case someone else has this problem.