Try go to your addons page: *'''Tools''' >> '''Addons''' >> '''Extensions''' look for this manager and try unistall it. *[https://support.mozilla.org/en-US/kb/disable-or-remove-add-ons Disable or remove Add-ons] Also: *[https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware Problems caused by malwares] *[https://support.mozilla.org/kb/reset-firefox-easily-fix-most-problems Reset Firefox]