Maybe you wish block miners websites that use your PC to mine virtual coins as bitcoin or monero etc and let your PC slow by overloading the processor.
In Chrome there are No Coin and Mining Blocker specific to block miners.
In Firefox you can use No Miners EN 0.5.4
Some days ago I see was self installed in my FF 56.1 the plugin Safe Browsing Version 4 (temporary add-on)
this temporary add-on enables the new version of the Safe Browsing API, which protects against dangerous and deceptive sites. It will be automatically removed once the roll-out of this feature is complete.