Because it’s not based on chromium(blink web engine), there are two other well supported web engines which browsers can be based on, WebKit (Apple), and Gecko (Mozilla).
At the end of the day, if it’s built on Blink, it’s liable to have Google break things they don’t like on the back end. Including ad blockers.
Opera used to be built on it’s own web engine (presto) but since 2013 it’s been built on Blink.
There are plenty of browsers built on Gecko that aren’t fire fox. So if you don’t trust Mozilla to build your browser, and don’t want your ad blocker bricked by Google, you have options.