I was just browsing the "Google News" page.
I clicked on an interesting looking headline.
My browser froze for a short time.
When the new tab opened, it was a page telling me I had an outdated version of Firefox.
The download window opened (without my consent), with a file that contained the word "Firefox" but was otherwise unfamiliar.
I closed the download window and the tab.
The download window opened again.
I closed it again.
"Help" + "About" reassures me that I'm up to date.
BE CAREFUL OUT THERE !