Please check these things first:

(1) Not blocking redirects (2) Not blocking cookies (3) Clear cache and cookies (4) Test with McAfee WebAdvisor disabled

Redirects

If you turned on the optional redirect block, that can cause failures on some sites -- often without any notification or with bizarre symptoms. Please check that here:

Windows: "3-bar" menu button (or Tools menu) > Options Mac: "3-bar" menu button (or Firefox menu) > Preferences Linux: "3-bar" menu button (or Edit menu) > Preferences

In the left column, click Advanced. On the right side, with the "General" mini-tab active, uncheck the box for "Warn me when websites try to redirect or reload the page" if it is checked.

(This is an accessibility feature intended for users of screen reading software and not a general-purpose tool of value to most users.)

Cookie Settings

This article describes how to relax cookie restrictions to see whether that helps with sites that do not function correctly: Websites say cookies are blocked - Unblock them.

Clear Cache and Cookies

When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(A) Clear Firefox's Cache

See: How to clear the Firefox cache

If you have a large hard drive, this might take a few minutes. If you do not see the number going down on the page, you can reload it using Ctrl+r to check progress.

(B) Remove the site's previously-set cookies (save any pending work first). While viewing a page on the site, try either:

• right-click (on Mac Ctrl+click) a blank area of the page and choose View Page Info > Security > "View Cookies"
• (menu bar) Tools > Page Info > Security > "View Cookies"
• click the padlock or "i" icon in the address bar, then the ">" button, then More Information, and finally the "View Cookies" button

In the dialog that opens, the current site should be pre-filled in the search box at the top of the dialog so you can remove that site's cookies individually.

Then try reloading the page. Does that help?

WebAdvisor

You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then cast a critical eye over the list on the right side. All extensions are optional. If in doubt, disable.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Any improvement?

I just noticed -- and thank you for sharing your extensions list -- that you have something called "Privacy Protector Plus" that I haven't heard of before. Does it allow you to make exceptions for sites where you do NOT want blocking? You may want to check for that before disabling any extensions. (In other words, before step 4 above.)

By the way, when I search part of that error message, Google tells me there are 267 results from the past year. When I look for ones not containing the word facebook, there are only 4. I don't know whether that means this is nearly certainly a problem using a "Log in with Facebook" feature or whether all those sites have Like buttons...

"redirect uri is not whitelisted in the app's client oauth settings" - Google Search (latest year)

I checked all the things you listed up above. Actually I have Firefox set to delete all history, cache and cookies whenever I close the browser.

After I had changed one particular privacy setting, it did NOT affect anything else (applications) on Facebook, only the one application/game. So I wonder IF there's an issue with the application and I should contact the developer?

I just loaded out that application again (after rebooting my computer a few hours ago) and I'm still getting that same error message. No other applications tonight are NOT being affected, same as earlier today.

Forgot to mention that I changed back that privacy setting when I started encountering issues with that one application. So have no idea what else could be causing it, except Facebook. There is a message that comes up when I close the error message box. The message says "Facebook connect".

Since I am on Facebook right now, don't think it should be telling me that. So guess I'll contact the developer of the game and tell them about my situation, unless you have other suggestions.

Thanks!

It does sound like the problem is limited to one site. The developer might have made and error, or Facebook might have revoked their access to your FB profile for some reason.

A friend (who is also a PC tech) helped me with the description of what that error message meant from your link.

But it ended up being something 'simple' and I did not think to check this area because of my not being a PC tech to understand the context of the description of the error message and how to resolve the issue (in computer language terms).

How I resolved the issue (after a suggestion by my friend and PC tech): I ended up removing the application/game from my Facebook app settings. When I removed the app, the box came up stating that if I continued all shared information, settings, etc. would be deleted. I then went back in and re-added the application/game and my problem was resolved.

I suspect that Facebook had made some changes to their platform and it only affected that one application/game. Or the game developer made a change. But whoever was responsible, by removing the app/game all traces of it were deleted. When I added it again, the permissions went back to the default settings and everything is working fine.

In hindsight, it had nothing to do with me tweaking a couple of security settings in Firefox. It was a unique situation that I made changes and after that the one game did not want to load out. So I had made the assumption the changes I made to security settings in FF was the culprit. Thankfully that ended up not being true.

Thanks for all your input, as it did help my friend (and PC tech) direct me in the right direction to resolve this issue.