With Firefox, especially with Firefox on Linux, which always had and still has poor GPU support, I frequently encounter sites that do not work well or they do not work at all. So I must keep a backup browser, which is normally Vivaldi, because typically any site that works in Chrome also works in Vivaldi.
Moreover, Vivaldi has a great advantage over both Firefox and Chrome, in it the command to print a Web page usually works fine, while in both Firefox and Chrome it almost never works correctly.
Both Firefox and Chrome are almost never able to render correctly a "printed" page, even if they render the same page perfectly on screen. In the printed page, the graphic elements have almost always wrong sizes, which results in overlapped or invisible page elements. I suppose that this is caused by the fact that many Web pages stupidly use element sizes in pixels, instead of using length units, e.g. points or inches or mm, and both Firefox and Chrome might scale pixels wrongly when rendering for resolutions that differ from that of the screen, while Vivaldi scales them correctly.
Besides the "Print" command, the second feature that I like in Vivaldi better than in Firefox or Chrome is that it accepts mouse gestures for most commands, as alternatives to keyboard shortcuts, so you do not need to move the hand from the mouse while browsing.