Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control>(Mac=<Command>)<Shift> A)

In the Add-ons Manager, on the left, select Extensions. Remove ******* Anything. Now do the same in the computers Programs Folder.

Windows: Start > Control Panel > Uninstall Programs. Mac: Open the "Applications" folder. Linux: Check your user manual.

You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

If you are referring to the Bing search engine that comes with Firefox in some localizations like en-US, if is one of the "as supplied" search engines and if someone would manage to remove it from the omni.ja file deep inside of Firefox, it would return with the next Firefox update. Mozilla has "protected" those "as supplied" search engines from being deleted or changed via Malware, which prevents the user from deleting them, too.

The best you can do is to hide it via the "Change Search Settings" feature in Firefox Preferences > Search.

https://support.mozilla.org/en-US/kb/change-your-default-search-settings-firefox Under One-Click Search Engines un-check Bing to "hide" it.