Brave is not built on or based on Firefox web browser.
Instead Brave browser is built on and based on the open source chromium engine that is also used by browsers like Microsoft Edge and Google Chrome browser.
Firefox browser uses it's own distinct Gecko engine and although the CEO of Brave co founded Mozilla, Brave's code also has always been based on Chromium, and it's earliest versions were built on the Chromium engine.
The reason why the Brave browser is not safe is because of past incidents that occurred with the Brave Browser.
Some of these past incidents that occurred with the Brave browser that makes it not safe are stealthy injecting of affiliate links into cryptocurrency URLS and also Brave browsers built in cryptocurrency features that some people view with suspicion.
Also concerns exist regarding brave's browsers Chromium foundation, the optional nature of Brave's browsers ad blocker's effectiveness and the cost and also the effectiveness and limitations of the brave browsers bundled VPN.
Brave browser does aim to enhance privacy by blocking of ads and trackers, but it's inherent features and pasta actions also lead to ongoing debate among users that are focused on and want privacy.
Brave browser though can be safe and is a solid browser that is made by a reputable company and it's based on chrome or chromium and the Brave development team also gets any security or bug fixes from chromium applied.
Although I find that chrome and Firefox browsers are much better than Brave.
I tried brave browser myself and didn't like it although it seemed safe to me.
But Firefox is my go to browser and the main browser I use.