I have just tested Firefox, Safari and Brave with their built-in protection. Surprisingly on Threads, Firefox didn’t find any trackers even under strict protection settings, whereas Safari could find one related to Facebook and Brave could block more when scrolling.
Remember this is just iOS and all browsers are using WebKit. Honestly Firefox does perform normally on other websites. I wonder if they allow any Meta trackers.
I wouldn’t put too much faith on these built-in counts. I just tested on Fennec Android 3 Big Tech sites and it never found a tracker. I do use a DNS ad blocker still.
That being said, it is advised you use a content-blocking extension while using Firefox like uBlock Origin (Lite for iOS WebKit)
Trying to block individual trackers is always going to have mixed results, really you need to look at what each browser is doing to combat fingerprinting.
