Browser mining is a good way of earning some coins if you have a heavy traffic website. If you are a visitor and if you are fine when a website uses your computing resources for mining thats OK otherwise it's not right.
Browser mining is indirectly abusing visitors computing resources.
Recently piratebay was found hijacking visitors web browser to mine cryptocurrency.
NoCoin extension is a simple chrome addon which blocks the browser mining.
Currently the addon is available for chrome browser only. Addon for Firefox is under development.
If you are interested in installing the addon, simply visit this page and click the link which says "Chrome web store" which opens the chrome web store link.