Trying to remove all search engines can have unpredictable consequences, as a user discovered in this thread: How do I set "browser.search.defaultenginename.US" to a null string or at least a website that doesn't exist to prevent searching from the URL bar?

But your preferred default should "stick" after you set set it. Is the problem that:

• Firefox won't save the change to the default (e.g., if you close and re-open Options it has changed back immediately)
• Firefox ignores your preference
• The setting changes back after you exit and start Firefox up again
• The setting changes back when you shut down Windows and start it up again?

Some reasons settings change might get lost include:

• Explanations in this article: How to fix preferences that won't save
• Programs like Advanced SystemCare with a browser protection feature that undoes settings changes between sessions
• Extension overriding your choice
• Damaged/tampered program files

On that last point, since a Refresh does not cleanse the program folder, try this:

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

(1) Download a fresh installer for Firefox 38.0.1 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(2) Exit out of Firefox (if applicable).

(3) Rename the program folder:

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\OldFirefox

(4) Run the installer you downloaded in #1. It should automatically connect to your existing settings.

Does a cleanly reinstalled Firefox search Bing normally??

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins

Try to delete the search.json file and possible search-metadata.json and search.sqlite files in the Firefox profile folder to reset the search engines to the default.

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Firefox will rebuild the search.json file from the default search engines in the "browser\searchplugins" folder in the Firefox program folder and the searchplugins folder in the Firefox profile folder.

You can delete possible user.js and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in the current Firefox release.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences