Either click the “hamburger” menu button on the top right, then the question mark button at the bottom of that window for the help menu, then “About Firefox”, at which point it will update and prompt you to restart. Otherwise, get the latest version here:
https://www.mozilla.org/en-US/firefox/new/
Why? A big vulnerability was found. Read more about it here:
http://www.tomshardware.com/news/firefox-security-vulnerability-upgrade-patch,29783.html
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