Tracking protection seem to prevent some sites from using Facebook login.
Disabling tracking protection for some sites seem to be the solution to enable Facebook login and sometime Facebook permission granting to 3rd part apps/websites. Is this a Firefox problem? Are tracking protection somehow disable pop-up blocker of Facebook?
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You can never be sure that if you use content blocking feature that you aren't blocking wanted content. If pages aren't working properly then you will have to disable this feature on that web page.
You can check the Web Console ("3-bar" menu button or Tools > Web Developer) to see what content is blocked.