Sounds very frustrating.

Regarding this part --

Sirrrus said

Something I have noticed, particularly when logging into Facebook is that as I am typing facebook.com, the drop down bar automatically shows m.facebook.com even after a clearing the cookies/history/etc.

-- you should not see suggested addresses from history after clearing history. However, you may see suggested addresses from bookmarks. Those usually are designated with a blue star on the right. If there's no blue star, then if you arrow down to that suggestion using the keyboard and press the Delete key on the keyboard, it should not be suggested any more. Make sure to edit the address line after that because it will still show the address of that deleted suggestion.

But... back to the main issue. Sites may redirect to a mobile version if they detect that you have a smaller screen. I wasn't able to make Facebook flip to mobile this way, but maybe this is affecting yours.

Firefox will report different screen sizes depending now how much you have zoomed the display. To see what I mean, you can play with this page:

http://dev.jeffersonscher.com/resolution.html

As you zoom (hold Ctrl and tap the + key), the "screen.width x screen.height" will show smaller and smaller values. This is Firefox's way of telling the site that even if you maximize the window, at the current zoom level, this is how much will fit. However, some sites may mistakenly believe you have a tablet or phone instead of a computer.

I'm using a 55" LED SmartTV. But the bookmarks thing is an interesting theory. I have searched my bookmarks and cannot find facebook.com, cnn.com, foxnews.com, nbcnews.com, mlb.com or any of the websites that come up as Mobile. However, those are also websites I have as bookmarks on my iPad and iPhone. But that's Safari...? How is my Windows desktop PC with Sync not activated pulling bookmarks from my other two devices?

Do you have some software on your iOS devices to sync with your desktop Firefox? I haven't looked for anything like that myself. If not, there should be no connection between your mobile Safari bookmarks and your Firefox bookmarks.

Your Firefox sent this forum the normal user agent string: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:40.0) Gecko/20100101 Firefox/40.0

While I'm struggling to think of what is special about those sites you listed, could there be another filter involved, outside of Firefox? For example, something in Avast or Microsoft's Family Safety parental controls filter?

As a temporary workaround, perhaps can you find a "desktop site" link on those mobile pages and use that to get the correct display...

How are your screen dimension reported?

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Try to boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) to see if that has effect.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/