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.

See also:

• https://support.mozilla.org/en-US/kb/i-found-fake-firefox-update

GaryVP said

Note - if you are seeing this page when you don't want to, update Firefox to the latest version Indefinite Articles Bellygraph Picobusiness Android Development Laboratory Google Mozilla Firefox

That is 'undefined.com', a redirect virus and a browser hijacker.

Go to the 3-bar menu => Addons => and look for any unfamiliar or suspicious looking extensions, like e.g.:

SafeSearch Incognito SearchAssist Incognito

When you find any : remove them.

Do the same in Windows' Control Panel => Programs.

However : most malware will be 'hiding' somewhere , so let's run malware scans :

Further information can be found in this article :

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners.

They all work differently - what one program doesn't pick up, the others might.

Also see : https://malwaretips.com/blogs/remove-undefined-com-redirect/