It's good you recognize popups as suspicious because yes, Firefox updates automatically. You can set Firefox to notify you of updates, but those look nothing like the current rash of new-one-every-day sites abusing the Firefox Logo to trick users into trusting their malware download. If you need to update to Firefox 48, you can use the About Firefox dialog on the menu, as described in this article: [[Update Firefox to the latest version]]. If you saw a phishing page, you can report that site as described in this article: [[I found a fake Firefox update]]. Most likely an ad on the previous page you were viewing redirected Firefox to the fake update. You could consider using an ad blocking extensions such as the following: https://addons.mozilla.org/firefox/addon/ublock-origin/