I use firefox on the steam deck, and am pretty pleased with the UX

You use touchpads on the Steam Controller or deck to move a mouse cursor and there are good gestures for scrolling. I use https://addons.mozilla.org/en-US/firefox/addon/gesturefy/ in Firefox to quickly switch tabs and other features without keybinds I struggle to remember or moving the mouse. I've used gestures exactly like this in firefox since 2004.

The keyboard using two trackpads is also pretty good.

What are you expecting to be different? What are you hoping for and why?

The real question IMO is what browser? Is it Chrome? Will DRM content work out of the box? Will high resolution streaming work?