The killer feature in Opera, compared to Firefox and other Mozilla products, is the ability to restore the content of all tabs at every browsing session; not just after experiencing a crash, but in a systematic way, whenever starting the application. I have finally found a Firefox plug-in that does that: Session Manager. Why isn't this a standard feature in Epiphany and Firefox?
Epiphany vs Firefox compatibility issues
Even though Firefox and xulrunner are essentially the same codebase, the different browser-agent is enough to make Epiphany unsuitable for everyday use. As I recall, Rosbeh and several others reported this as being one of two reasons for shipping Firefox as their default browser, the other being a question of brand recognition: Firefox had the full-page ad in the New York Times and is multi-platform, while Epiphany is virtually unknown and GNOME-specific.
This must be fixed.