Preventative Measure

See /questions/981475#answer-516884 downthread

Using the sample site John99 mentioned, I created a script to defang that page and others that use similar techniques. You can add it to your Bookmarks Toolbar for future use in the unfortunate event that you run into one of these pages. 

http://dev.jeffersonscher.com/bookmarklets.html#escape  
Also see explanation and screenshots  /questions/981475#answer-516977 

Advice on what to do if you get this sort of attack. Speaking from looking at the page I got from the now removed link in the Original post.

BTW the site exists still from the link in the OP it is however not showing in Google searches that I tried.

Here is the fix dead simple solution

• You may want to carefully note the full information in the address bar .

1. Try to close the tab once only
2. You will get a popup use the mouse to select the [Leave Page] button but do NOT left click.
3. Now use the keyboard key [Enter] (or [Return]) after a second or so it should auto repeat hopefully rapidly enough to clear the problem in a few seconds. The rogue tab will then close
4. It may then be a good idea to clear the rogue site from the History. Use the forget option.
   Remove websites from the address bar suggestions_clearing-all-items-for-a-single-site

• Note the rogue site is likely to identify your country and send information appearing to come from your country or a multi country official body. Europol EC3 or FBI for example
• You may wish to copy and paste the address from the location bar. It may be useful if you wish to report the problem. Should you report this on a site replace all dots with the word dot.
  (for instance v88634.com as v886341(dot)com and s845340.com as s845340(dot)com )

For info a current one I see is

http://europol.europe.eu.id974784510-4458260206.s845340.com/?flow_id=8614&414304=33302/case_id=46449  

The site does

• Scare people using some information that looks correct and some that is plausible. Impersonating police or similar sites.
• Appears to lock up the browser
• Demands and presumably collects money with a 12 hr deadline.
• Does do some sort of validity check on the cash voucher

What does NOT work

• Following most of the advice about Malware.
  Because you do not have malware installed on your computer.
• Resetting or re installing Firefox
  Resetting and reinstall normally leave the session store information alone.

What is not worth trying

• Reinstalling the Operating System
  That is overkill
• Blocking the fbi site
  https://www.fbi.gov/ or https://www.fbi.gov/ They are genuine. Firefox may give you a warning, as they have security issues !!
  (A known problem Bug 863517 - https://www.fbi.gov/ has active mixed content (JS and CSS) that are blocked by the mixed content blocker )
  Or the Europol site
• Using the popups and clicking one at a time. The popup floats over your browser and will disappear with each single click. The file I have takes over 70 clicks.

CARE
Some superficially similar warnings may be from malware that does encrypt your files, or otherwise damages your System

• e.g. see http://blogs.technet.com/b/security/archive/2013/11/19/ransomware-is-on-the-rise-especially-in-europe.aspx

I will mark this as the solution to this problem as it will solve the issue.