Vivaldi definitely has a learning curve. It’s great once you have it set up how you like (which, granted, is way too time consuming for the average user). But the tab stacking and tiling is so immensely useful for me, I can’t use other browsers without missing those features now.
I’m not sure what it is. I suppose this is the case for the heavier web-applications, but the average website (which is where my expertise is, not actual applications) also feels slightly worse on FF. And as far as I know, I don’t use any chrome-specific tricks or optimizations.