Many site issues can be caused by corrupt cookies or cache.

• Clear the Cache and

• Remove Cookies
  Warning ! ! This will log you out of sites you're logged in to.

Type about:preferences<Enter> in the address bar.

• Cookies; Select Privacy. Under History, select Firefox will Use Custom Settings. Press the button on the right side called Show Cookies. Use the search bar to look for the site. Note; There may be more than one entry. Remove All of them.
• Cache; Select Advanced > Network. Across from Cached Web Content, Press Clear Now.

If there is still a problem, Start Firefox in Safe Mode {web link} While you are in safe mode;

Type about:preferences#advanced<Enter> in the address bar.

Under Advanced, Select General. Look for and turn off Use Hardware Acceleration.

Poke around safe websites. Are there any problems?

Then restart.

Done all of that (and again, just now). Problem persists.

Oh, WOW! NEW ERROR! I went to a website called Subeta and went to their Wardrobe (version 3!) and I'm experiencing the same error!

Here's the link directly to the image: http://i.imgur.com/7QQbhGt.png

Load the web page. After the page is loaded, hold down the <Shift> key and press <F5> to reload the page.

One, SHIFT + F5 opens the Performance tab, and lets you record the performance of... I don't know, but that's what that does.

Forcing the page to reload is, obviously, considering what all I have done, is something I have done over, and over, and over again. Assume that if there is a guide on anything Firefox isn't displaying or loading properly, I have done it, if not a few times over to make sure I performed each step properly.

That's why I'm here. Everything I have tried so far has been unsuccessful. I noticed the error after I added a second user to this computer, and that's where the problem originated from. During all of my attempts to fix the problem, the "error" affected both Firefox(es). Today, I cleared out and removed the second user, hoping it would fix the problem. It didn't.

Now that another very specific page is throwing this error, it makes me wonder if, for some reason, Firefox and my computer together are not handling a specific type of encoding and instead throwing this odd error.

Are there any other non-standard approaches to fix this problem? Could it be that something in my computer isn't letting Firefox read a certain type of code? (It seems to be the stylesheet, but I'm unsure), yet allows other browsers read it just fine? That's what my hang-up is, why is it Chrome can read this page, for example, but in Firefox I wouldn't even be able to type in this box?

I do thank you for trying to help, however. (I realize I may sound terse, but I'm just tired.)

Hmm. Look on your address bar for the reload symbol. It looks like a round line with an arrow on it. Hold down the Shift, and click the circle.

Sorry that I am just now getting back to you, I have been away due to surgery.

As stated before, that does NOT work. It isn't a cache issue, it's some sort of software issue. I have cleared cookies, removed the program, deleted whatever remained OF the program as guides suggest, and the problem still persists.

What security software do you have?

There is security software like Avast and Kaspersky and BitDefender and ESET that intercepts secure connections and sends their own certificate or that incorporates special web shielding features that can block content. See also:

• https://support.mozilla.org/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

I'LL BE DARNED, THAT WAS IT. All I had to do was shut off the Shields Control and BAM! Everything was normal again!

Thanks so much, cor-el! The guide worked perfect and everything is back to normal!