Vivaldi’s toolbar can be customized just like Firefox, but you additionally also get a bottom bar and a sidebar to place toolbar buttons on.
Vivaldi has a Spotlight-like search bar you can open with F2 to quickly find a page in your history or type any browser command like hiding the UI. You can also string multiple commands together and add them as a toolbar button.
You can add websites to your sidebar too to open them in a slide-out window of sorts (basically the same thing as Opera GX’s sidebar).
You can tile multiple tabs to open them in a split or grid view, which I haven’t found a way to replicate on Firefox so far.
And as someone else already mentioned, I personally find installing CSS and JS mods to be a lot more accessible on Vivaldi.