Type "about:config" into the URL-bar and hit enter. Confirm to the dialog that you'll be cautious and then search in that page for "xpinstall.signatures.require

Type "about:config" into the URL-bar and hit enter. Confirm to the dialog that you'll be cautious and then search in that page for "xpinstall.signatures.required". I'm guessing, that this is disabled for you. If it is, and you want to get rid of the McAfee-extension, then remove the McAfee extension and re-enable xpinstall.signatures.required. It shouldn't be able to reinstall then. If it does anyways, check about:config, again, if that preference is re-enabled. If it is re-enabled, either some add-on in your add-on-list is malicious and re-enabling it, which then allows McAfee to reinstall, or McAfee is abusing the prefs.js-file in your Profile-folder ([0]), which can be used to set about:config-values. All in all, that this is an about:config-toggle to begin with, is kind of crap, as it makes this sort of stuff possible. Mozilla has plans to eventually remove this about:config-toggle from standard Firefox and instead provide a separate unbranded version of Firefox which then does still have that toggle for people who want to use unsigned extensions, but for now, this change has been put on hold, because things have been going fine, and it would just mean additional organizational chaos as well as people, who don't know about the unbranded version, screaming about Mozilla creating a walled garden. But if all of the above applied for you, you should probably report this to Mozilla, as that might be enough reason to get them moving on this change. [0] https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data https://support.mozilla.org/en-US/kb/profiles-where-firefox-...