looks like rendering adblockers extensions obsolete with manifest-v3 was not enough so now they try to implement DRM into the browser giving the ability to any website to refuse traffic to you if you don't run a complaint browser ( cough...firefox )
here is an article in hacker news since i'm sure they can explain this to you better than i.
and also some github docs
It was always more of a marketing statement than a security one. And then it was copied over and over because most sites don't write original articles, they just remix other articles. Firefox is as safe as any software - there will be bugs and security issues but so will in Chrome and I wouldn't say the rate of new bugs and security issues will be much different between the two projects.