I don’t know whether FF lacks these features, but I’m using:
- webpage splits
- search by image
- go to non-<a> url in bg tab
- open in new tab in a virtual sub group rather than just to the right or at the end
- tabs retain width on close until mouse goes away (helps with closing series of tabs)
- bookmarks open in new tab by default
- last tab doesn’t close the browser
- gestures and toolbar customization
That’s in Vivaldi, I probably forgot a bunch of features that feel natural but may have no FF counterpart. Tbh, looking at FF settings, there’s basically none. You can’t miss features that you never had, I guess.