I found no issues with delta dot com. See attached pic. However, it did take a number of seconds for their server to respond and perhaps provide me with permission(s) to view their site..

So there may be something blocking Delta on your end.

My suggestions are to temporarily change the mode of operations of FireFox from regular to safe, aka FF safemode. Then retest the webpage.

Next suggestion is to temporarily change the mode of operations of Windows from normal to safemode. Then reboot and retest the webpage.

If Delta dot com then works for you under one or both of the modes above, you have eliminated the possible causes.

Yes, the Delta website worked when I opened it in a "private window". Will I always need to do that or is there a setting I can change to make it work in normal mode?

Private windows differ from regular windows in a few ways:

(1) Cached files from regular windows are not re-used in private windows.

You could clear the web cache and see whether that helps in the regular windows:

How to clear the Firefox cache

If you have a large hard drive, this might take a few minutes.

(2) Cookies set in regular windows are not re-used in private windows.

To remove Delta cookies, while viewing a page on the site in a regular window, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that.

In the dialog that opens, you will see one or more matches to the current address so you can remove the site's cookies individually without affecting other sites.

(3) Some extensions might be disabled in private windows.

When you are viewing your extensions on the Add-ons page, the ones without the purple mask icon do not run in private windows. More info in this article: Extensions in Private Browsing.

That might point toward an extension interfering with the site in regular windows.

Now the PNC Bank website tells me to use a different browser. Please tell me what settings to change to use these websites properly. I did not have this problem before updating Friefox to the latest version.

Users have been reporting problems with the PNC Bank website for a while now. It's possible the bank is tired of getting support calls from Firefox users and that's why they posted that message, I don't know. Here are the other threads for reference:

https://support.mozilla.org/search/advanced?sortby=1&a=1&w=2&q=pnc