You could install a extension like uBlock Origin to help block the bad ads.

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 server 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.

You can find the full version of the current Firefox release (47.0.1) in all languages and all operating systems here:

• https://www.mozilla.org/en-US/firefox/all/
• https://www.mozilla.org/en-US/firefox/notes/

Technically what you're getting isn't "spam", which is a term related to receiving unwanted email. https://en.wikipedia.org/wiki/Email_spam

If you want to report it, use Help > Report deceptive website ... while the pop-up is open, to report the URL to the SafeBrowsing database so the URL can be added to the blocklist which is used by Firefox, Chrome, and Safari.