Why it is not recommended to install browsers as flatpaks?
I don’t use chrome or chromium based browser, so it doesn’t affect me?
Or i still better not use browser as flatpak? If so, why Firefox have official flatpak?
And if not using flatpaks, than how to prevent potential browser exploits?
It conflicts with the browsers own sandboxing and Flatpaks aren’t necessarily created by the package or distro maintainer. You are placing trust in an intermediary.
I can’t say for sure, but I will note that just because chromium is generally regarded as having better sandboxing than Firefox and friends, that doesn’t mean they don’t have sandboxing. I would still expect any modern browser to have some degree of sandboxing, and I would almost certainly expect it to be better than what Flatpak offers. (I am by no means well-versed in this area, though.)
Keep your browser up to date, minimize extension installation/use, and don’t click things you can’t trust. If your threat model demands more than that, a Flatpak probably isn’t going to help you much. I’d say start using VMs/Qubes/Trivalent via secureblue (and follow the previous advice regardless).