Some things technically not part of the core feature set of a browser (at least not the one you've mentioned), yet even just one of them missing would make me immediately drop Firefox:
- End-to-end encrypted tab, history, and bookmark sync across devices
- Content translation, as mentioned above ideally in a privacy-preserving way
- Plugin support
- Cookie-jar-per-tab and proxy-per-tab support (Firefox allows doing both through Multi-Account Containers)
It's almost like a "one size fits all" browser can't actually exist. I'm hoping for a more minimal, but excellent, browser to come around to meet the needs of the sort of user I am.
I think this is one of the adverse effects of Firefox changing how extensions work. In the Good Old Days, Firefox was a reasonably minimal browser and you could pick and choose which advanced functionality you wanted by choosing which extensions to install. But since extensions have been largely neutered now, a whole lot of that advanced functionality has to be built into the browser proper. This is fine if you want that stuff, but it's really uncomfortable if you don't.
- End-to-end encrypted tab, history, and bookmark sync across devices
- Content translation, as mentioned above ideally in a privacy-preserving way
- Plugin support
- Cookie-jar-per-tab and proxy-per-tab support (Firefox allows doing both through Multi-Account Containers)
I'm sure some other users also feel the same.