Let’s do a full clean re-install;

Note: Firefox comes in three or more folders on all computers. They are;

Maintenance: (Programs Folder) <Windows Only> Firefox itself: (Programs Folder) And two folders in the profile of each user on the computer for each Firefox profile for that user.

If you remove the Firefox folder, the user profiles would not be affected.

Download Firefox For All languages And Systems {web link}

Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86) Note: Check Both Folders

Mac: Open the "Applications" folder. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it. See Install Firefox on Linux; https://support.mozilla.org/en-US/kb/install-firefox-linux

If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory. http://www.mozilla.org/firefox#desktop ++++++++++++++++++++++++++++ Look for, and remove any Mozilla or Firefox program folders. Do not remove the Mozilla Thunderbird folder if there is one.

Do Not remove any profile folders.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. +++++++++++++++++++++++++++ If there is a problem, start your Computer in safe mode and try again.

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode This shows how to Start all Computers in Safe Mode; Free Online Encyclopedia

Thanks very much for the instructions. Before I go ahead, I wanted to confirm.

Here are the folders that I've found:

C:\Users\jkerman\AppData\Mozilla C:\Users\jkerman\AppData\Local\Mozilla C:\Users\jkerman\AppData\LocalLow\Mozilla C:\Users\jkerman\AppData\Roaming\Mozilla C:\Program Files\Mozilla Firefox C:\Program Files (x86)\Mozilla Firefox

I assume that I should delete these. Two more that I wasn't sure about:

C:\ProgramData\Mozilla C:\Program Files (x86)\Mozilla Maintenance Service

Should I leave these or delete them?

Then, I think the steps are: 1) Reboot computer 2) Run registry scanner (I'll have to look up info about this) 3) Install (Firefox Setup 66.0.5.exe)

Does this sound right?

You appear to have both the 32-bit and 64-bit Firefox version.

• (64-bit Firefox) "C:\Program Files\Mozilla Firefox\"
• (32-bit Firefox) "C:\Program Files (x86)\Mozilla Firefox\"

The main Firefox profile folder with your personal data is in C:\Users\jkerman\AppData\Roaming\Mozilla, so you lose your data if you remove this folder.

The Mozilla Maintenance Service is used to install updates in the background.

• https://support.mozilla.org/en-US/kb/what-mozilla-maintenance-service

OK. I installed the new version of Firefox, and again I see the same problem-- disk usage zooms up to 100% and my computer basically freezes up.

I then tried restarting my computer in safe mode, and here, too, running Firefox seems to stop everything. (When I was in safe mode, I was unable to get Task Manager started so I don't know exactly what the level of disk usage was, but I effectively couldn't do anything.)

Is there something else I need to do?

Many have reported issues using the 64bit Firefox. Install the 32bit version from; Download Firefox For All languages And Systems {web link}

Let’s do a full clean re-install;

Note: Firefox comes in three or more folders on all computers. They are;

Maintenance: (Programs Folder) <Windows Only> Firefox itself: (Programs Folder) And two folders in the profile of each user on the computer for each Firefox profile for that user.

If you remove the Firefox folder, the user profiles would not be affected.

Download Firefox For All languages And Systems {web link}

Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86) Note: Check Both Folders

Mac: Open the "Applications" folder. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it. See Install Firefox on Linux; https://support.mozilla.org/en-US/kb/install-firefox-linux

If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory. http://www.mozilla.org/firefox#desktop ++++++++++++++++++++++++++++ Look for, and remove any Mozilla or Firefox program folders. Do not remove the Mozilla Thunderbird folder if there is one.

Do Not remove any profile folders.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. +++++++++++++++++++++++++++ If there is a problem, start your Computer in safe mode and try again.

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode This shows how to Start all Computers in Safe Mode; Free Online Encyclopedia

Do a malware check with several malware scanning programs on the Windows computer.

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  https://www.malwarebytes.com/
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  https://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  https://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  https://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  https://support.kaspersky.com/viruses/disinfection/5350

https://support.kaspersky.com/viruses/utility

Thank you cor-el and FredMcD!

I ran several of the scanning programs. Malwarebytes: turned up nothing. AdwCleaner: removed PUP.Optional.Legacy SuperAntiSpyware: found and deleted 14 Adware.Tracking.Cookie Microsoft Safety Scanner: turned up nothing Kaspersky Free: deleted 2 objects; quarantined a couple (It looks as though the ones deleted were part of TIBCO.) TDSSKiller: turned up nothing

I don't know that any of these were really significant or a factor in the behavior I was seeing. I tried Firefox again, and still had the same problem.

So, next I followed FredMcD's advice and I installed the 32-bit version of Firefox. This worked!! I don't know why there's a difference, but this version is OK.

I really appreciate the help I got with this issue!

As I had said,

Many have reported issues using the 64bit Firefox

I am glad to hear that your problem has been resolved. If you haven't already, please select the answer that solves the problem. This will help other users with similar problems find the solution more easily.

Thank you for contacting Mozilla Support.