Aww, I feel so bad for you ......

You've tried just about everything in the book, to no avail. The only thing left to do (that I can think of) is to Refresh Firefox - would you be willing to do that  ?

https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Start your Computer in safe mode. Then start Firefox. Try Safe websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

I have been tracking this thread for a while, and I am sure that eventually, someone is going to want a solution. In addition to FredMcD's message, I would strongly suggest doing all of the scans in safe mode (For malware).

Two things that adware/browser hijacker can cause.

Despite the obvious issues, it can be hard to remove it in normal mode. Safe mode allows for a clean Windows boot to potentially solve the issues. I know you have done some steps, and it might be tedious, but the following is a thorough malware check up:

1. Restart your PC in “Safe mode with networking.” 2. Install and run RKill to kill malicious processes and services 3. Check your Programs and features and see if there are any new recently installed programs that you don’t recognize. If there are, remove them. 4. Check your task manager for any suspicious processes, if found, identify folders and try to remove them manually. Or just "Win key + R" and type %appdata%. Afterward, delete potentially malicious folders. 5. Do a full scan with anti-virus software of your choice or use Windows Defender to clean up initial infections. 6. Scan your PC with Hitman Pro, Malwarebytes, and AdwCleaner. Multiple anti-malware solutions will confirm that the threat was removed. 7. Clean up your Registry and Cached files with CCleaner 8. Restart your PC in normal mode and do an additional scan to confirm that the malware is gone.

I don't know how, or why, but this issue simply resolved itself a few days after my most recent post in this thread.

I ran every anti-virus and anti-malware I could find, and even paid for a couple new ones, and none of them found anything.

I cleared cache and the cookies, and lost most of my logins, (but I had made a written copy of all of them beforehand, so it was only infuriatingly inconvenient), and did everything else without totally destroying the functionality of the browser, with no changes to the issue. I would not "do a refresh" as the last time I did that, I lost all my bookmarks, page settings, and most of my passwords.

Here's the inexplicable part:

A few days after my most recent post, I was entering some grades in my school spreadsheet, and on a whim, tried to do a search from the "New Tab" page (the one that shows page thumbnails) and *poof* it was as if it had never happened.

I have no clue how it fixed itself.

Sometimes the problem goes away, and we never know what happened. Please flag your last post as Solved Problem so others will know.

These can't get your data back, but will help in the future.

These add-ons can be a great help by backing up and restoring Firefox

https://addons.mozilla.org/en-US/firefox/addon/febe/ FEBE (Firefox Environment Backup Extension)

FEBE allows you to quickly and easily backup your Firefox extensions, history, passwords, and more. In fact, it goes beyond just backing up -- It will actually rebuild your saved files individually into installable .xpi files. It will also make backups of files that you choose.

https://addons.mozilla.org/en-US/firefox/addon/opie/ OPIE

Import/Export extension preferences

FEBE never manages to restore FF to it's correct configuration. It is better than nothing, but it does not work well for me.

Home Page http://softwarebychuck.com/febe/febe.html Firefox Environment Backup Extension Home Page

Can you describe the problem? I never had an issue. In fact, several times I lost my passwords (no idea what happen) and Febe restored them no problem.