Recently Firo, former Zcoin had a 51% attack and it was possible to change/tamper data of 306 blocks(or so). What exactly is that? Is it like the guy who had control of 51% of the network had overwritten the block? How does anyone get benefitted from such an attack? What's the step to prevent such attack?
https://cointelegraph.com/news/privacy-focused-firo-cryptocurrency-suffers-51-attack

For the Firo team, the attack was only possible because the project was yet to deploy Chainlocks on the mainnet. A Chainlock is a secondary validation layer that reportedly
mitigates a 51% attack.

According to Yap, with Chainlocks activated, an attacker would need to control at least half of all Firo master nodes in addition to the usual 51% mining hash rate dominance.

Firo has reportedly completed the testing protocols for Chainlocks and is primed for its full deployment in the next few weeks. When integrated, Chainlocks will be the latest protocol
feature added to the project after activating the Lelantus upgrade.