Me too re: wanting to like Vivaldi. I still use Firefox... I have tried to use Vivaldi as a daily driver, but (on Linux) it still frustrates enough to send me back to Firefox. The scrolling speed is way too slow (as on other Chromium variants), and addons only partly fix it. It also is nothing close to the glassy smoothness of Firefox when using a touchpad ("MS Precision" type, though again in Linux).
The Vivaldi UI is not any more set in stone than the Firefox one, though. It uses .css to style it the same way Firefox does, and it is possible to use user stylesheets to change the UI. It is not the kind of thing a "regular" user would do, but neither is doing the same on Firefox.
I have used .css with my Vivaldi (partially my work, mostly the work of others in the Vivaldi forum) to get the UI to look pretty much identical to my Firefox, which in turn looks like Firefox before the Australis misadventure, with the titlebar at top, then menubar below, then the URL/nav bar with discrete search field, then the tab bar (with new tab button at left and close tab button at right, in keeping with Fitts' law and the principle of important UI elements being in a predictable, fixed location), then the content, then the status bar with the addon icons on the right side.
With the ability to style the Vivaldi UI, you can pretty much do whatever you want with it, just like Firefox. On Firefox, I use Aris-T2's custom CSS to make the bookmarks toolbar into a status bar (and it is possible to add the bookmark toolbar functionality to a second row of the URL bar, if desired... a neat workaround around Moz's decision to nix the status bar "because Chrome doesn't have one."
Mozilla got rid of the unread tab state because Chrome doesn't have one (Vivaldi added it back in, where on Firefox or Chrome, it requires an addon now).
Mozilla got rid of the option to not fire a select-all on a click on the URL bar (a single line of code provided that pref, and they removed it with the excuse that it reduces the maintenance burden on Mozilla, even though one person agreed to be the maintaner for that line of code to remove that burden). The official Moz response was that this is how other browsers (read: Chrome) do it, and that "all" operating systems behave that way, so they are just making Firefox behave consistently.
My problem is that my OS, OpenSUSE Tumbleweed with KDE Plasma, does not behave that way, so in their zeal to force Firefox to behave consistently on "all" OSes, they made it behave inconsistently on the one I use, which is the only one that matters to me. On top of that, it should be up to the user whether the browser behaves consistently with the OS. What's it to Mozilla if I prefer my browser to act differently than my OS?
There have been countless other such options that used to exist, that I used to use, before Mozilla ripped them out. This started long before the Quantum leap backward, when Mozilla removed the defining feature of Firefox (XUL addons that could do anything) in favor of... of course, Chrome's weaker addon setup. But with the XUL addons, I could put back anything Mozilla took away. It was always a sure bet that if I bristled when some important feature was removed, an addon to correct the issue would soon exist.
With Quantum, of course, there are a lot of things addons cannot do to fix the UI. They can't truly add an unread tab state... the workaround is to have an addon change the tab title of an unfocused new tab, and then to style the tabs that have had their title changed. It's hacky, and should be unnecessary, but... Mozilla. Addons can't create an actual status bar (like the classic addon Status-4-Evar did)... instead, another hack to repurpose an existing bar has to be what we settle for.
I imagine that someday Mozilla will lop off the ability to use userChrome.css, and on that day Firefox will be dead to me. They already hid it behind a pref, which is what Mozilla usually does before deleting a feature completely. They're also angling to remove the option for having a discrete search bar (because Chrome doesn't have one), which is another big one for me.
Vivaldi is far from perfect, but it is not removing features and trying to be a minimalist browser (like Chrome) the way Firefox is. Firefox keeps losing features, while Vivaldi keeps adding them.