Vivaldi also has a “window panel” that is basically a tree-style list in your sidebar of all your tabs across all windows and workspaces, and recently closed tabs and sessions.
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.