Don't trust the pop-up, trust your Help > About box.

Did you get that pop-up after visiting a specific site, or did you always see it no matter where you go? If you are seeing it everywhere, you may want to scan using free anti-malware tools. Try tools such as:

• MalwareBytes' Anti-Malware
• AdwCleaner (for more info, see this alternate AdwCleaner download page)
• See Troubleshoot Firefox issues caused by malware for more information

You can also try add-ons such as Adblock Plus, and check your pop-up settings. Please see Pop-up blocker settings, exceptions and troubleshooting for more information.

Actually it may have been a legit update notice from Mozilla but by mistake. Firefox 33.0 is planned for Release on October 14.

From https://support.mozilla.org/en-US/forums/contributors/710762 Tyler Downer wote: Due to a manual error, we accidentally released Firefox 33b9 to release users today. None of them should have been able to actually install it, but they would have gotten messages saying they needed to update. We've fixed this, and due to an already planned upgrade to our release software this won't be possible in the near future.

Sorry for the confusion guys.