I don't do facebook, but a quick search found this. https://www.reddit.com/r/facebook/comments/xf3fb0/facebook_comment_popup_window/

Yes, I found that before I made this post. It's a wonky work around which I'm sure the great minds at FB will will change so that doesn't work. I'm look for a more permanent fix that will make FB work the way it used to. This popup does not make FB easier to use. It does just the opposite. FB doesn't know when to leave well enough alone. It's just like when they went to the "New Look" a year or so ago. That was another change that was unnecessary.

Have you contact FB support about this?

Facebook Support? Have you ever tried to use FB Help Center? They don't want to hear any complaints about the user experience of their products.

But thanks for the suggestion.

As this is coming when FB is used -then this is FB caused not Firefox caused here. Also using older version is asking for security risk for Firefox Browser itself since newer version fixes bugs or issue with older Browser securities.

D'oh!