Vivaldi is a great power user browser, which is generally what the Linux audience is. Firefox is pretty good for power users too and it takes the cake when it comes to privacy and security, but Vivaldi just has those exclusive features that you just can not replicate on Firefox.
It's somewhat like Firefox used to be, cuztomizable UI and all that... a lot of menus with UI tweaks that just make your browser your own and make your life easier... it brings back what was taken from us when FF made some drastic changes.
How is it more customizable than Firefox? Last I used Chromium based browsers, stuff like TreeStyle Tab was impossible besides a hacky separate window whereas extensions in Firefox are able to make those drastic changes to the UI.
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.
These features are why I prefer it over firefox, but I am curious about how it will be affectes by Manifest V3, if it losses things like an adblocker and dark reader, witch I doublt, them I will need to use waterfox, but even then, firefox, on phones and specially tablets its way worse
Customization wise, a lot. Speed wise, none at all (it's slower any way you slice it). Compatibility wise (with websites), the same as Chrome, everything works.