Feanor

I am curious, how have you reached that conclusion?

Well, besides the benchmarking that is a difficult topic, I guess the "extensions" versus "hard coded features" is a matter of philosophy.
Personally, besides the fact that FF is open source, I like more the extensions because it means I don't have to wait the core developers to decide about adding a feature but anybody from the "community" can "extend" the browser.
You see ADBlock and Firebug came earlier than "block content" and "dragonfly" on Opera and being developed separately, they are more sophisticated, like pretty much any other extension. Plus, I don't need to load the full set of extensions, can have multiple profiles that each load a different set of tools, like "common browsing", "development", etc.
I don't see the point in having "JS debugging" built in when 99% people who use the browser don't even know what it means.

I see at this topic from the opposite direction. Opera does not have the option of extensions so it is FORCED to add built-in features to stay competitive with FF, while FF just have to care about the general framework and then let the "community" create all the needed add-ons.
Same goes for Chrome, for example.

The mail client... FF originates from a split off of the all-in-one Mozilla browser and it was decided at the beginning to strip off the browser all the unneeded features. But, among all the Mozilla/Gecko derivatives the all-in-one browser still exists and its name is Seamonkey.
Again, what is the point in having a built in mail client when few people is going to use it anyway?

For common people who don't care much of technicalities Opera is good like any other browser. There must be a reason why Opera's market share is so low but I am not a marketing expert.

Opera's model is safer.

FF extensions are actually a nightmare of poor coding, bugs and exploits. But I guess that is somehow inevitable if you want to open your software to "community" contribution. It is impossible to review the code of hundred or thousand extensions, despite they are published over AMO. You give extensions the power of adding real features to the browser, then they can do pretty much anything once installed. So while FF itself is relatively safe because the core crew spends lots of time in bug fixing and security updates, you cannot tell about FF with some extension installed. Same goes for performance, FF has got a nice memory collection feature but then a single extension can eat your RAM or slow down everything.
But I guess the same "open" mechanism that provides poor code can be self-regulating in some extent, because when an extension is bugged there are chances somebody else notice that and reports to the author and even provides patches or better code.

From a different perspective, being "open" FF was able to attract and involve an huge community of users and programmers, who are both useful for code contributing, design, bug reporting, testing, extensions AND as evangelist, promoting FF everywhere they can (family, friends, school, workplace, etc). I can like Opera but I will always be just an user, so I am not personally motivated in advocating Opera over other browsers.