Please try to remove history, cookies, temporary data and site preferences.

Okay, I did that. It had no effect on the problem with the Comments section of usatoday.com

The "good" news is that all the tabs that I had open are gone and my History section is blank.

So here's a new question/problem to go with my original: Is there a way I can recover those tabs?

I feel sorry for that... you should have read very carefully the instructions page. Removing the history is removing the pages you visited and the last session:

   Browsing & Download History: Browsing history is the list of sites you've visited that are shown in the History menu, the Library window's History list, and the Search your bookmarks, history and tabs with the Awesome Bar's address list. Download history is the list of files you've downloaded that are shown in the Use the Downloads window to manage downloaded files.
   Form & Search Bar History: Form history includes the items you've entered into web page forms for Control whether Firefox automatically fills in forms with your information. Search Bar history includes items you've entered into Firefox's Search bar - Easily choose your favorite search engine.
   Cookies: Cookies - Information that websites store on your computer store information about websites you visit, such as site preferences or login status. This includes information and site preferences stored by plugins such as Adobe Flash. Cookies can also be used by third parties to track you across sites. For more info about tracking, see How do I turn on the Do-not-track feature?.
   Note: In order to clear cookies set by Flash you must be using the latest version. See Updating Flash for instructions.
   Cache: The cache stores temporary files, such as web pages and other online media, that Firefox downloaded from the Internet to speed up loading of pages and sites you've already seen.
   Active Logins: If you have logged in to a website that uses HTTP authentication since you most recently opened Firefox, that site is considered "active". Clearing this logs you out of those sites.
   Offline Website Data: If you've allowed it, a website can store files on your computer so that you can continue to use it when you are not connected to the Internet.
   Site Preferences: Site-specific preferences, including the saved zoom level for sites, character encoding, and the permissions for sites (like pop-up blocker exceptions) described in the Page Info window. 

That data it's gone unless you have a backup of your profile.

We encourage all users to perform a backup of their profiles before trying every support action. You can find how to backup and restore your profile here .

About the main question: maybe the webmasters of Facebook or that sites are working on a feature that hides part of the comments so they don't use so much space on the screen. And it seems it doesn't work well. All you can do is ensure Firefox is correctly installed, without any extension that may change your browser experience, no ad blocker is installed and your antivirus software is working as you want.

Still feel sorry for you. Cleaning cookies and personal data usually solves many particular site issues because it forces them to fully download again and if, for example, a buggy change was made on a site and now has been fixed, maybe Firefox still loads the buggy version. By cleaning cookies, Firefox has to download again the site with the last working version.