Multi-account containers are a feature I use tirelessly. Standardized add-ons are great too, so people can release one add-on and hit multiple browsers. I also really like Firefox sync for sending bookmarks/tabs between devices. Firefox is also extremely fast nwadays, and comparatively easy on system memory.

These are things that have improved in the last decade. I also like the fact that FF isn't owned by an Ad company that pushes binaries supposedly built from an open-source project controlled by an Ad company.