Sorry, this is indeed a known problem with Firefox 55 (and later, as far as I know).

Assuming you don't want to switch to an older version at this point, a possible workaround is to change the table layout just a bit using a bookmarklet. If you use Print Preview and see the problem, you can cancel out and run this and then check again.

There is a one-time setup:

(1) Select and code this long line of code:

javascript:void((function x(){tbls=document.querySelectorAll('table'); for (var i=0; i<tbls.length; i++) {tbls[i].setAttribute('cellspacing', '0'); tbls[i].style.borderCollapse='unset';}})());

(2) Create a new bookmark in your preferred location (e.g., Bookmarks Toolbar or Bookmarks Menu) by right-clicking and choosing New Bookmark

(3) Paste the script into the Location field

(4) In the Name field, you can put something like Table Border Print Hack or whatever you like

(5) Click the Add button

Then you will just need to click the button to apply it to pages with tables when they need it.

Does that work for the pages you need to print?

Depending on page styles, preview may show the left or right side border missing after this change (e.g., this test page: the test page), but the border should appear in the actual printout.

Related thread: Printing changes from page 2

Actually, that site is a little odd. You first have to click the Print Current Page link, then cancel the print dialog, click the bookmarklet, then print manually (not clicking the link again), for example, using Ctrl+p or the Print button.

Are you kidding me? If I have to be a computer programmer to make ff work for me I will just use a different browser. I have two others that work fine.

Yes, you should use a different browser for that site if you need to print from there a lot. Definitely.

But you do not need to be a computer programmer to create or use a bookmarklet, and they're remarkably handy. Maybe you could try it some day.