The mining only took place for several hours on February 11th before Texthelp disabled the plugin to investigate. Government sites like the UK's Information Commissioner's Office also took pages down in response. As with most of these injections, your system wasn't facing a security risk -- you would have just noticed your system bogging down while searching for government info. The mining goes away the moment you visit another page or close the browser tab. The biggest hassle was for the site operators, who are now discovering that their sites are vulnerable to intruders slipping in rogue code without verification.
It's not certain who's behind the attempt, but these hijacks tend to be the work of criminals hoping to make a fast profit.
The big problem: this might continue to happen for a while. Although antivirus tools can catch Coinhive, a more definitive solution would be to use a fingerprinting technique (subresource integrity) that verifies of outside code and blocks any modifications. And there's no indication that many websites, whether government or private, are in a rush to implement it.