hello, the only legitimate site to download firefox from is mozilla.org - firefox also contains an automatic updater, so you don't need to download any file to update, you can just go to the firefox menu ≡ > help ? > about firefox in order to trigger a manual search for updates.

if you are getting loads of popups of that sort it might also be a sign of malware already active on your system:

1. reset firefox (this will keep your bookmarks and passwords)
2. afterwards go to the firefox menu ≡ > addons > extensions and in case there are still extensions listed there, disable them.
3. finally run a full scan of your system with different security tools like the free version of malwarebytes and adwcleaner to make sure that adware isn't present in other places of your system as well.

Troubleshoot Firefox issues caused by malware

Mozilla has their own network system for downloads of Firefox and other Mozilla products/projects as Mozilla does not use or need any third-party download sites for downloads or updates.

The only exception was when releases.mozilla.org was made up of accepted mirrors from around the world before they switched to cdn which you can can see with say http://download.cdn.mozilla.net/pub/mozilla.org/firefox/

https://blog.mozilla.org/it/2012/08/03/dear-mozilla-mirrors-thank-you/

. Whenever you get a message / popup that software / files need to be updated;

DO NOT USE ANY OF THE PROVIDED LINKS

While this may be a legitimate message, it could also be Malware or a Virus. Any time you want or need to check for upgrades, go to the web site of the True Owner of the program in question. For example, to check out Firefox, go to Mozilla.org. {web link}

You can sent a report to The FBI via their web site Internet Crime Complaint Center (IC3) {web link}

You can also report such a site at; Google Report Phishing Page {web link} which is the same when done while on site by going to Help > Report Web Forgery

Help us safeguard Mozilla’s trademarks by reporting misuse {web link}