When I switched to Vivaldi I felt like it's 2003 again, and I've just switched from IE to Firefox. Every single thing Mozilla removed from Firefox over the years is here, and most of the stuff I used hacky addons that would often break is here too! In the core browser, as first-class features, without the need to fiddle with userChrome.css or look through obscure flags. It really is a breath of fresh air and it puts into perspective how many excuses I've made for Firefox over the years. It's not worthy of being my browser, simple as.
Mozilla took my fundamental addons that separated Firefox from other browsers, they took my RSS reader, they took my cool Torrenting and Email clients that were a part of the browser itself. The TreeStyleTab requires you to go through obscure and hidden config files that often break with updates and the extension itself is not stable and fiddly. On top of that, I had way more Firefox extensions that aren't even different from Chrome extensions in major ways. In Vivaldi, I just get a nice panel with RSS, Calendar, Translator, Email client, Notes, whatever I want! The adblocker is built-in, the privacy features are built-in, you even get to put your tabs wherever you want. It has theming support that is as good as Firefox Colors, and it has custom search keywords that replace DuckDuckGos bangs for me more often than not. It even has the dark mode among other page filters, a screenshot tool, web page tiling! All the things that would turn my Firefox profile into a slow extension pile that barely works and longs for death.
Mozilla's "goals" of removing key features meant for people who actually would want to use a "google alternative" are laughable, and it's as bad on "privacy" axis as Chrome is because you have to use something like LibreWolf to get the actual privacy from it, very much like you have to use ungoogled-chromium with Chrome. If they think that turning the browser into a Chrome clone with some bumper stickers that say things like "Proud not to use Blink" and "We do say privacy a lot", then it's already dead to me.