Check if "Mozilla Maintenance Service" is installed. You should be able to find it's folder under Program Files, next to Firefox itself. You can uninstall that service (uninstaller executable is inside the folder) without uninstalling Firefox itself.

First, Type about:preferences#advanced<enter> in the address bar. Under Advanced, Select Update.

[v57+] Type about:preferences#general<enter> in the address bar. Select Update.

Select Never Check For Updates.

Also turn off Use a background service to install updates

Install Older Version Of Firefox {web link} Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release {web link} ESR Notes System Requirements

Thanks for the responses, I'll report back later.

Hello,

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.

Thanks to Vindicar for providing the solution. You other guys need to try to read stuff before posting.

Vindicar said

Check if "Mozilla Maintenance Service" is installed. You should be able to find it's folder under Program Files, next to Firefox itself. You can uninstall that service (uninstaller executable is inside the folder) without uninstalling Firefox itself.

Nothing like that on my computer. Running Win 7 Pro. As usual, the easy answer never works for me. I should know better. Off to the repair shop.