The browser Maxthon was rumored to run a mining program in the background while their browser was open years before
this announcement was made.
No, OP is running Firefox, Web Content is a Firefox process and is
not linked to any website you have opened in a tab, it is the rendering process that transforms HTML and CSS into computer graphics and it also runs the Javascipt engine. For performance reasons there is usually one Web Content process running on each core. That's why a bunch of tabs crash if you kill it.
In short, there is nothing wrong with your computer OP, there is no mining software ruinning on it and you don't need to reinstall anything, it's just that the casino websites are built with heavyweight Javascipt that uses a lot of CPU power when you use them, also the way how some casino designs look attractive.