You mean OS as in "operating system", not "open source", which made me question how you'd suggest Safari and Edge before I realized what was going on.
I don't know why using the default browser a OS comes with or not would be better? Who cares? All that matters for me is what features it has and that performance isn't horrible, and if I cannot have vertical tree-style tabs in it and hide the tab-bar, then I personally have no use for it. So stuck with Firefox I am, and been for decades.
Besides, if my OS is Arch Linux, what browser is my "OS-browser" anyway?
> I don't know why using the default browser a OS comes with or not would be better?
Are being ignorant on purpose now?
Apple's Safari has integration with Apple's Password app which firefox does not. Edge automatically connects with your Microsoft account.
Both browsers sync tabs, bookmarks and so on across your Microsoft/Apple account connected devices.
For a lot of 'average' users this is already extremely convenient.
On top of that, Safari is still the most 'energy efficient' browser you could use on macOS.
That's usually said as Open Source Software (OSS) or longer acronyms that sound like dental care.