Users of Firefox are reporting instances whereby they may be browsing a legitimate site (such as ebay), when the browser automatically opens a new tab with a Firefox branded page and a title of ‘urgent Firefox update’, download now.
Needless to say the update contains malware which will infect your computer, so do not download or run it.
Scammers have in the past infected websites with their malware, but this appears to be different in that it is spread through code embedded in the advertising, so in effect could pop up anywhere.
if in doubt, click on Help at the top of the browser window then About, and it will tell you if your browser is up to date.
You can read more about it here: https://support.mozilla.org/en-US/kb/i-found-fake-firefox-update
I already clicked on it, what should I do?!
Ensure that you have up to date virus protection running, and perform a full scan, removing any infected items.
Note that note all virus protection is equal, if you feel it is likely that you are infected try running scans with several different providers. In a test we did, Windows Defender didn’t pick up any infection, whilst Malware Bytes did.
It’s not all about you
If you work in a team, remember to ensure that all members of your team are aware of threats and how to deal with them, if you share data it only takes one infection to potentially create a serious problem for your business.