One browser cannot fulfill all the needs. Each browser has its unique features. It all depends on the "Out Of Box" Experience. Practically a non technoholic user will not bother to check if any adons are available for the perticular browser or not. Many are not even aware of browsers other than IE ( no wars pls ).
I have been using Opera for the past 5 yrs and it has given me the best results. But Opera is not the only browser which I use. I also use Firefox as well as IE Pro.
Opera -
Pros: Super fast. Tiny footprint. loads and loads of features like integrated Main/chat/RSS client, Mouse Guestures, Awesome zoom feature, Speed Dial
and not for forget a super download manager with bittorrent protocol support to name a few.
Cons: Most of the cons are site related, some sites dont render properly.. but its basically the site developers mistake and not Opera's. Closed Source, but it doesnot matter if you are not a developer.
Firefox:
Pros: Second widest used browser. Fast, No problem rendering sites. Quick bug fixes, Loads of addons. Open Source, this too doesnt matter if you are not a developer.
Con: Heavy on resources. Pathetic download manager ( but can be improved by using adons ). Auto Update doesnot work with Linux ( thats my experience ), Unsigned addons can be used with FF this may have a security risk if the add-on contains malicious code so use unsigned add-ons at your own risk.
IE: Not in the topic of discussion.. so no comments.
So at the end if you are looking for out of box experience then its Opera all the way No browser can match the features of opera.. but if you want tons of add ons and if you are ok with your browser taking up resources then FF.