Firstly I must Point out That the problem with Firefox is That the XulRunner engine although allows for great customization, it is slow.
Opera Doesn't have Plugins like Firefox has, however it has just about every feature built into it as well as a bunch of widgets you can get.
Opera comes with a E-mail Client, IRC client, News-reader, RSS reader.
For Web Developers who use Firebug and FirePHP and are being rediculously stubborn saying they can never switch, think again, Opera Has Dragonfly a great source analyzer, Debugger.
Opera is an acquired taste, Personally, I despised it at first, but now I swear by it.