I'm not the OP, but Vivaldi has been my main browser for many years now.
The reason why some people like Vivaldi is the same reason why other people dislike it. It has a lot of additional features and customization options that other browsers don't. You may find that cool (e.g., people who used old Opera), or you may dislike it, because "I just want a browser to open web pages."

But anyways...here are some features that I really like and I miss in other browsers:

Highly customizable shortcuts, gestures and command chains (macros) I use mouse gestures a lot, and on Firefox I had to install an extension to get that feature. Also one tiny feature that I love in Vivaldi, that I really miss in other browsers is to switch tabs by scrolling mousewheel while the cursor is over the tab bar.
Easy way to add custom search engines (I assume other browsers have this too, but I know that on Firefox it's a little bit longer process to make one)
Many ways to organize tabs (stacking, grouping, renaming tab groups...)
Tab tiling (arranging opened webpages in a single window, good for comparing stuff or multitasking)
Mail client and RSS feed reader (not very polished but it's still convenient)
Workspaces (good for separating tabs, e.g., work, shopping, entertainment...)
Simple markdown notes (you can access them quickly from a side panel, and u can quickly add selected text from a webpage by right clicking the text and add to note)
Customizable menus (e.g., customizing options that are presented in the right click context menu)
Quick commands (it's like a command palette from which you can search history, bookmarks, run commands, do simple calculations, etc.) you could in theory make your browser UI-less and just use the Quick commands.

Those are just some of my favorites but there is a lot more...And almost all of these additional features you can disable selectively if you wish to do so.

[–] Kissaki@feddit.de 3 points 1 year ago

Easy way to add custom search engines (I assume other browsers have this too, but I know that on Firefox it’s a little bit longer process to make one)

The reasonable alternative in Firefox is search bookmarks. Create a bookmark with search url target and %s placeholder, and give it a search keyword. Then you can search with keyword search text.

[–] Flaky@iusearchlinux.fyi 2 points 1 year ago* (last edited 1 year ago)

Vivaldi-to-Firefox here with a little insight! Firefox addons can have the permission to hide tabs, and there are addons that take good advantage of this. Simple Tab Groups can essentially replicate Vivaldi Workspaces, as well as Sidebery if you want something a bit more on steroids in the form of a sidebar.

~~While I've got you here, I've had issues with Vivaldi not being able to block Google search results I don't want to see. Might've been me not setting my blocklist up properly, but it works on Firefox with uBlock Origin and Safari with Adguard.~~ Seems like Vivaldi doesn't support some of the more advanced filtering that Letsblock.it uses for that - AdGuard works.