Hi effgo, If the malware corrupted or the added to the Firefox files then a clean reinstall may be the answer. Indeed in the answer you linked to that did work.

If for some reason you do not wish to try a clean reinstall, it is probably worth testing what happens doing a custom install of a second Firefox with an additional Firefox profile and a separate install location, but that is even more complicated. An easier test may be simply to do an install of Firefox Developer edition, that should work and by default takes care of the install location and profile.

• From: https://www.mozilla.org/en-US/firefox/channel/#developer

What is your reason for not wishing to try a clean install ? It is a phrase we use internally on these forums. The procedure is designed not to lose your settings and information. I have included details of how to do this at the end of this post. Resetting or Refreshing Is a user friendly way of making some profile changes. As you are knowledgeable enough to move files between Firefox profiles you can use more advanced & targeted methods of troubleshooting or resolving problems.

You mention copying files from a clean profile. Presumably you have tried a clean profile.

• Does a clean profile work ?

If a clean profile works it may be better trying copying files to a clean profile rather than overwriting files in the current profile.

• See Recover important data from an old profile

The question did not contain full troubleshooting information.

• Is there a file user.js present in your profile ?

If so and you did not create it yourself it would be interesting to see the content of that file. It is likely to be hidden by default and possibly the .js extension may not display

Clean Reinstall

Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove your Firefox program files and then reinstall Firefox . This process does not remove your Firefox profile data (such as bookmarks and passwords), since that information is stored in a different location.

To do a clean reinstall of Firefox, please follow these steps: Note: You might want to print these steps or view them in another browser.

1. Download the latest Desktop version of Firefox from mozilla.org (or choose the download for your operating system and language from this page) and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (or open the Firefox menu and click the close button ).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

More information about reinstalling Firefox can be found here.

WARNING: Do not use a third party uninstaller as part of this process. Doing so could permanently delete your Firefox profile data, including but not limited to, extensions, cache, cookies, bookmarks, personal settings and saved passwords. These cannot be easily recovered unless they have been backed up to an external device! See Back up and restore information in Firefox profiles.

Thank you for chiming in, John99.

Creating a clean profile didn't work. A new installation of the developer edition (my first on that system) is free of the issue.

Why I do not want to do clean installs? Because with Firefox it is easy to restore what Refresh leaves intact anyway and extremely hefty to restore all other things i.e. addon configuration and about:config preferences. Those other things comprise the vast majority of my customization to Firefox. The first is an utter mess when it comes to organization, including folder naming. With the second, Firefox marks ANY altered setting as 'user set'. Including those altered by addons or... malware.

The user.js (deleted now) content is: user_pref("extensions.autoDisableScopes",14);

Hi again!

Refreshing Firefox did not help. Clean re-installation of Firefox did help. Addons and addon configuration is gone, as expected. I will mark the answer as the solution of course as probably loosing those data couldn't be avoided after all.

... Addons and addon configuration is gone, as expected. ....

With luck we can get all that back pretty quickly, as long as you have not been deleting things.

The new install worked. The new profiles did not. So the problem appears to be with the program files rather than the profile. You should be able to use the original profile. Proceed as follows

1. Create a new profile. Give it some meaning full name such as recovered-2015, and use the location suggested.
2. Run Firefox with that profile. Then check its location and name using about:support {Troubleshooting Information) -> Open Folder {Or whatever the option is worded as). Note the additional xxx...dot prefix in the full profile folder name. (Not seen in the profile manager's options)
3. Now replace the newly created profile. Retain the folder name and path, but delete the content and replace with that of a copy of the original profiles folders and files.
   • You will have the files needed for copying on your Windows desktop, as a result of using the Refresh option. (It is the topmost folder that needs renaming, or the contents being carefully copied to the empty location. The full hidden name; of the newly created profile; needs to remain the same as when it was created.)

The prefs.js file was probably fairly innocuous. It is probably worth noting that in Firefox profiles it is probably better practice to disable files by renaming them rather than by deleting them. See also

• https://mike.kaply.com/2012/02/21/understanding-add-on-scopes/
• http://kb.mozillazine.org/Starting_your_Mozilla_application_with_a_specified_profile