I've attached a link to an image below.

Facebook works super poorly in Firefox these days so this probably is a legit addition, but the box also extends out the right side of the column and breaks the grid.

The text reads:

Your ad blocker is blocking posts from friends

Certain ad blockers can stop websites from working properly. You can pause your ad blocker on Facebook to allow the posts to load.

There's a blue button which says "Fix now" and it just walks you through how to do it with another message:

How to stop your ad blocker on Facebook

Click the ad blocker icon on your browser. From the drop-down menu, turn it off or select the option to not run it on pages for this domain. It will still run as usual on other websites.

Is Facebook actually withholding posts from my friends? Is it just a hostile black mail tactic they're playing with me?

Screenshot 1: https://postimg.cc/vD7JjDbj Screenshot 2: https://postimg.cc/8fCmty1T