I would like to point out that it is not all about memory usage. We must also take in consideration standard compliance(Opera has an advantage here), customization, stability and speed. Speed does not just depend on RAM usage. It is also about the way in which the browser renders web sites(the engine).

This is my version of the browser battle:

RAM usage: Opera wins! Uses a fraction of what IE, Chrome, Safari and Firefox use.

Standard compliance: IE does not even support half of the web standards according to Acid 3 Test. Firefox is quite standard compliant and renders almost perfectly. In fact the latest unstable release got a score above 90 when tested on Acid 3. Opera is even better than Firefox when it comes to standard compliance, while Safari and Chrome succeed in passing Acid 3 Test. Winners: Apple Safari and Google Chrome

Usability: Internet Explorer 8 comes with cool features such as accelerators, previewing all tabs, improved search and Web Slices. Opera 10 features an improved Speed Dial, graphical preview of tabs and finally spell check for blogs and forums. Firefox 3.5 has an updated clear private data tool, the Library(for quicker access to bookmarks, history and tags) , features a better awesome bar(which is my favorite killer feature and I miss in other browsers) and also has the ability to open a recently closed window. Safari 4 has some new cool visual features which make it worthwhile to use this browser. I don` t like downloading files without using Chrome because of its cool download interface feature which does not waste my time to switch between windows (or tabs in Opera) in order to open a downloaded file. Chromes address bar makes it easier to search. All these browsers have useful features but in my opinion Mozilla Firefox wins the feature battle.

Stability: IE is stable. Opera is also stable. Chrome as well. Firefox is stable. But Safari crashes quite frequently on Microsoft Windows. On Mac OS X it runs great. To be honest in my own experience Opera is the most stable of them all.

Customization: Chrome does not have an Add-on manager although there are add-ons available for Google Chrome. Opera is not so customizable. Safari cannot be extended with add-ons on Windows. Firefox can be customized more easily than IE because its interface is written in XUL. Undoubtedly Mozilla Firefox is the most customizable of them all.

Although my test isn’t reliable either, you have to admit Opera is more than Firefox out of the box. It is a fully featured browser with mail support and RSS reader. If you take Firefox and stuff it with custom-made (and I most of the time “leaky”) addons, the amount of RAM used will be a LOT more…

Opera 9.5b2
Build Numbers
Windows: 9945
Mac: 4789
UNIX: 1933

Known Issues
[Bug 325677] POP mail download doesn’t work on web.de (and some other servers)
[Bug 324727] The Octoshape plug-in causes Opera to crash on start-up
[Bug 322553] The “Execute program” action fails in certain circumstances

Changelog (since the last snapshot)
Fixed Bug 324643: “Remove from toolbar” doesn’t work
Fixed Bug 324645: Right-clicking an IRC channel prevents it from showing new messages
Fixed Bug 324678: Mail attachment options don’t work on Windows
Fixed Bug 324696: The “Go to Web Address” action fails
Fixed Bug 324699: Crash when choosing “Open Opera” from the tray icon after hiding Opera on Windows when the mail client is disabled
Fixed Bug 324700: “Save image” context menu item doesn’t work
Fixed a crash on start-up for some POP mail users
Fixed a problem where inline find highlighting didn’t work correctly in mail (or when using “white-space: pre-wrap”)
Fixed incorrect label on “Remove” button in newsfeed views
Re-added the “Delete mail” action for backwards compatibility

it isnt the fastest, it never was the most customisable, and it no longer has the smallest mem footprint.