That's not a real Mozilla site -- check the address! Unfortunately, anyone can steal an image off the internet and make a convincing fake site these days. There has been about a year-long attack on Firefox and Chrome users with fake update notifications that download malware. Very frustrating because Firefox's/Chrome's built-in bad site blocking feature can't keep up with the rapidly changing "phishing site of the day" technique used in the attack. Since you know that Firefox updates itself, it's best to avoid all purported patches and fixes offered by websites or popups, and use the internal updater when you think you might be falling behind. This article has the steps for that: [[Update Firefox to the latest version]]. The fake updates seem to be spread through advertisements on popular websites. Since the ad networks can't seem to weed them out, you could consider using an extension that blocks ads in web pages, such as [https://addons.mozilla.org/firefox/addon/ublock-origin/ uBlock Origin]. If you ever do accidentally open that .js file, please run Malwarebytes Anti-Malware and then the other tools in the support article: [[Troubleshoot Firefox issues caused by malware]]. If the infection isn't easily removed, the article also lists specialized forums where they can walk you through the use of more advanced tools. And finally, thank you for checking. More info: [[I found a fake Firefox update]].