Let’s do a full clean re-install; 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) Mac: Open the "Applications" folder.
https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac Linux: Check your user manual.

Do Not remove the profiles folder.

Look for, and rename any Mozilla or Firefox program folders by adding .old to them. Do not rename the Mozilla Thunderbird folder.

After rebooting the computer, run a registry scanner. Then run the installer. If all goes well, remove the OLD folders when you are done.

If there is a problem, start your Computer in safe mode and try again.

Starting The Computer In Safe Mode;
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Thanks for the reply. It is clear to me that I can remove all of Firefox and start all over. My question was whether I could just remove ver 38 with the Windows uninstaller, leaving ver 47 intact. Renaming and removing a bunch of folders and running a registry scan -- whatever that is -- sounds like a lot of work.

If you use the uninstaller, there is a chance it could remove both programs. By doing the above, you would clean out and install the current.

A registry cleaner removes bad entries from the registry.