Hi, it's a fake update designed to infect you with malware! You should NEVER EVER click on anything like that! Please see the article - [[I found a fake Firefox update]]. If you are in any doubt about the authenticity of update pop-ups etc., it's always best to ignore them and check for genuine updates yourself, by opening the 3-bar menu > Help (question mark at bottom right) > About Firefox. Alternatively, you can press the Alt key to bring up the main menu bar > Help > About Firefox. More - [[Update Firefox to the latest version]]. We are trying to find a permanent fix, but it's proving very difficult. A possible workaround which seems to be quite effective, is to install an ad-blocker, such as - https://addons.mozilla.org/en-US/firefox/addon/ublock-origin Or, but this may be less effective - https://addons.mozilla.org/en-US/firefox/addon/adblock-plus Any further relevant info you may have, can be posted on this contributors' forum thread - https://support.mozilla.org/en-US/forums/contributors/712056?last=69507 ''If your question is resolved by this or another answer, please take a minute to let us know. Thank you.''

There is indeed a legitamate Firefox update as in Firefox 51.0.1 from www.mozilla.org or www.mozilla.org/firefox/all/ and by internal software update in Firefox. The way Firefox does internal patch updates has not really changed much since the Firefox 1.5 days over eleven years ago.