I came across this article on ZD Net when I was reading about what cryptocurrency was all about because my PC would become slugish at times for no reason at all. I just wanted to give all of you a heads up info on what some web sites are doing when you visit them. I copy and pasted a portion of the article below. I installed a Block Coin Miner addon to my browser.
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Your PC may be used to find cryptocurrency when you visit websites, with or without your consent.
A new report suggests hundreds of websites have taken The Pirate Bay's lead and are now using visitor PCs to mine cryptocurrency without the consent of users.
A month or so ago, torrent search website The Pirate Bay raised concern among the community as visitors noticed their CPU usage surged whenever a page was opened.
At first, some worried that malvertising or embedded malware was at play; however, when the domain's operators realized the game was up, they admitted the power surge was due to a "small experiment" in cryptocurrency mining.
Cryptocurrency, such as Bitcoin or Monero can be "mined" and acquired through computational power. If enough PC owners donate power, then mining can be a way to rake in revenue.

CoinHive and JSEcoin are currently the most popular scripts being employed to hunt down cryptocurrency, and Adguard estimates that these domains have earned roughly $43,000 in a three-week period at little or no cost.
It has been estimated that The Pirate Bay may be able to make roughly $12,000 per month from mining cryptocurrency, due to the domain's heavy flow of traffic.

Just run U-BlockOrigin

Or No script

Either will block the mining. The first is the option of choice, as it is a well supported, constantly maintained, much more versatile addon