Yes, considering the surface area they are relatively secure. But the surface area is absolutely massive and I have no doubt in my mind that there are multiple

Yes, considering the surface area they are relatively secure. But the surface area is absolutely massive and I have no doubt in my mind that there are multiple full remote code execution zero-days in all major browsers. Treating security as an absolute is rarely useful because very few things are completely secure. No browser will be completely secure for the foreseeable future.