It's pretty terrible. The post and comments are positioned for a left and right sidebar and when the sidebars runs out or are excluded from the selection, Firefox collapses the main column into the left margin. Annoying.

There are some hacky ways ways to make it better for printing, but the details are fidgety so consider this experimental.

Step 1: Ctrl+click (or middle-click) the post date to launch it into a new tab as a permalink. This places it at the top of the feed.

Step 2: Open the Web Console in the lower part of the new tab. Either:

• "3-bar" menu button > Web Developer > Web Console
• (menu bar) Tools > Web Developer > Web Console
• (Windows) Ctrl+Shift+k

Step 3: Select and copy this script, and paste it into the command line of the console (starts with ») and press Enter to run it.

// Improve printing of FB posts
// Find the division marked class="permalinkPost"
var plp = document.querySelector('.permalinkPost');
if (plp){
  // If we found it, find the "main" ancestor node
  var main = plp.closest('[role="main"]');
  if (main){
    // If we found it, fix the positioning and widen it
    main.setAttribute('style', 'right: 0 !important; width: 100% !important');
	// Hide the sidebar to the right of it
    main.previousElementSibling.style.display = 'none';
	// Hide the top blue bar that prints on every page
    document.querySelector('#pagelet_bluebar').style.display = 'none';
  }
}

The first time, Firefox will ask you to do something to prove you understand it can be dangerous to run scripts from strangers. (It's true!)

If you need to print frequently, it would be nice to find a cleaner and quicker solution, but it's very late here already, so...

OK, I just tried that and it had no effect.

There are a couple of more things I have tried since yesterday, with mixed success.

1. Add-on "Facebook Post Screenshot". On one post it worked perfectly. On a second post it accidentally left one of it's own processing graphics overlaying the post. On a third post, it failed to complete.

2. Add-on "Print Edit WE". It sometimes works, but only if:

  a. I view the post in the timeline, not by itself.
  b. With the add-on, select just that post & comments.
  c. Use Delete Except - Unrestricted.
  d. Tools: Fix Page Breaks

Neither method works all the time for any Facebook post.

Hmm, possibly the permalink method didn't work on the post you were trying to print, or maybe that is only for company posts -- there weren't any personal posts in my timeline with more than 1 page of comments to test on. If I get time, I'll try to find another example to test on.