Opera vs. Firefox is a holy war that pits those who believe browsers should include lots of built-in functions against those who argue for a relatively small core browser with the option of having lots of add-on components to add functionality. This argument roughly parallels the MonlithicKernelvsMicrokernel? debate, with proponents of Firefox arguing that it is more important to focus on the core browser and leave add-on functionality to add-on authors. Opera fans, on the other hand, argue that it is troublesome for people to have to go out and find add-on modules for functionality that "ought" to be in the browser itself.
This religious war has died down somewhat recently as Firefox has integrated additional functionality (like spell checking, and advanced tab management features).
April 2010: Is this even an argument any more? Firefox appears to have stomped Opera into the dirt. Perhaps this page is a DeletionCandidate?
May 2010: No. No it's not. Google Chrome has left both behind in the dust. I eagerly await FirefoxVsChrome?.
January 2014: With Opera switching to Chromium/Blink engine, the previous 4-years-old note is funnier in hindsight. OperomeVsFirefox? rises again.
Is it possible that all three of the browsers mentioned here are craprods awaiting a spanking from some as-yet-unknown browser entry? Firefox in particular has been pissing off the long term user base for a while now by taking away user control over functionality and forcing certain usability features into the ashcan. Opera has been making their browser behave in a way that doesn't jive with anybody else's interpretation of a "proper" UI. Google has made Chrome sick with its crApple rendering engine and Google-only facilities. All of these turds need to be replaced by something that is W3C compliant, open, and truly user-configurable. We're waiting.