>Firebug/Web Developer – Opera Dragonfly
This looks like it has a lot of potential, though I hope they do more than create a Firebug clone. Competition is good for all of us end users. When Dragonfly is fully released, I expect it to be extremely useful for mobile testing.

>HTML validator – there’s talk to add it to Dragonfly, for now I just use w3c validtor
I sure hope they add it to Dragonfly; real-time validation is something I would hate to be without. W3C’s tool is nice, but it’s a lot less useful when doing a bunch of testing on non-cached pages in an admin area.

>Opera also basicallty have chatzilla, thunderbird, stylish, and foxtorrent built-in
It’s funny because those are probably the features most people find very useful and will be a good pull to get Opera more usage, but they’re the ones I care the least about.

>On a related note, the add-on stuff is not exclusive to Firefox, IE actually has even more extensibility than Firefox,
Well, you know what they say about polishing a turd

Thanks for the info, loligoth; it’s been a while since I last gave Opera a try. Maybe I’ll take another look at it in the near future.

For Opera :
GreaseMonkey – Opera built-in userjs support
Firebug/Web Developer – Opera Dragonfly
Live HTTP headers – nope, but then I just use web-sniffer which is much more versatile
HTML validator – there’s talk to add it to Dragonfly, for now I just use w3c validtor

Opera also basicallty have chatzilla, thunderbird, stylish, and foxtorrent built-in, and it has widgets too, and still has smaller memory footprint and installer size than vanilla Firefox.

For me it’s just like the micro vs. monolithic OS kernels debate, theoretically micro kernels are better, but still I find nothing wrong with monolithic kernels like Linux.

On a related note, the add-on stuff is not exclusive to Firefox, IE actually has even more extensibility than Firefox,