According to theverge a zero-day flaw was exploited
Vulnerability description
How risky this vulnerability(ies)?
Mozilla said, attacker would be able to inject malicious code into your system through malicious web pages if you visit them while running an unpatched version of Firefox[engadget.com]
How to update your firefox browser?
Click the Tab menu -> Help -> About Firefox -> it will start checking if may new updates and will download the latest version automatically, then a restart button will appear after. It wll be updated after restart.
Firefox direct download link: https://www.mozilla.org/en-US/firefox/new/
Thread-topic posted on Beginners & Help
Stay safe always....