Only Firefox’s extensions can fully integrate. This has a good and a bad side. If the extension is well build, it can become exactly like built-in. If the extension is bad-written, then you can have deeper problem. Chose wisely your extensions on Fx and you’ve a Full Native Fx with many built-in-like features.
Both, Chrome’s and Opera’s extensions don’t give many rights which means you can’t have such a good quality ones with whatever features you like.