That can happen, depending on the security setup of your computer. Please make sure you are running the latest version of Firefox.

Are you using the Mozilla Maintenance Service to install updates in the background? *https://support.mozilla.org/en-US/kb/what-mozilla-maintenance-service If you get a pop-up message asking to update Firefox or plugins or scanning for malware then such a message is likely a scam and you should NEVER respond to such an alert to avoid getting infected with malware. *only update Firefox via "Help -> About" or by downloading and installing Firefox from the Mozilla and never via a pop-up or link on a web page. *plugins should only be updated via the plugin itself or by visiting the home page of the plugin. See also: *https://support.mozilla.org/en-US/kb/i-found-fake-firefox-update