How to get rid of booby-trapped pop ups to update flash player?
When weird things happen on my clients' Macs, the first thing I do is run the free Malwarebytes' Anti-Malware (MBAM, formerly AdwareMedic software to see if any of the known "bad guys" is active on the system. As often as not, they are.

Also, have a look at your /etc/hosts file through Terminal:


Open Utilities > Terminal (or use Spotlight)
more /etc/hosts (press space bar to scroll a screen at a time)


See if fplayer.adobe.com is listed there. If so, we'll discuss how to remove it.

Try that and report back. Then we'll try step two.