While there is cross-pollination between macOS and iOS Macs are general purpose computers. And while it is true you cannot, like most PCs, crack open and customize a Mac and that to date most games are developed for PCs and not Macs as a side-effect of this (game development and hardware/graphics card development are incestuous), Macs are extremely capable computers that can meet demanding needs.
I have used Macs and PCs all my life. At this very moment I am playing Assassin’s Creed Odyssey in full pretty mode on a PC while my Mac in the other room renders a film and I type this message in an iPhone. I prefer using my Mac over the PC for most things - and certainly the Mac is where my bread is buttered - my PC is a fancy launcher for Steam.
Without being impolite, it sounds like you looked at the Mac, but didn’t bother to actually use it for anything practical, tasks at which it excels.
This explicitly wasn’t a question about Firefox at all, or even about what browser to use. It was just about Arc.
And before you get your knickers twisted tighter, I use Firefox (as my primary driver on Windows and my secondary on Mac). I am using Firefox right now, in fact.