Firefox:
- Multi-account Containers extension is amazing. I only use it for a handful of accounts, but it is very simple to use. This is probably the feature I miss most in Vivaldi.
- The interface integrates better with my DE (Cinnamon), and overall feels less cluttered. Vivaldi has some kind of bar on each edge of the window. Toggle UI (Ctrl+F11) helps to hide this, but Vivaldi does take more space away from the webpage.
- I use uBlock Origin for adblock which seems to block more than Vivaldi by default. Vivaldi's built-in blocker is really great though, and I haven't felt the need to install an extension for adblock in Vivaldi.
Vivaldi:
- I love the recent addition of a second tab bar[0], though I was skeptical at first. I can quickly stack tabs by host to declutter, and then see all the tabs on a second row with full details when I select the stack.
- The experimental builtin RSS[1] and Email panels are wonderful. Still some rough edges, but it is nice to not rely on another tool or service for RSS, and it is my favorite RSS reader I have used so far because it is simple and stays out of my way.
- Many more tab management features. Saving tabs as a session, tiling, stacking, periodic reload, etc. Not all features I use daily, but handy when I need it.
Overall, I've settled on using both browsers now. Each has strengths and weaknesses and I will use whichever helps me best in the moment. I'd say give it a try, and don't think about it as a switch, rather as a new tool.