I've been using Firefox since its inception, and its glorious introduction of tabbed browsing. However, every new update, Firefox seems to get ever slower, and use more memory and more processor.
Finally, last week, after Firefox hung yet again using 100% of processor and over 300mb of memory, I decided to give Opera a go. So far, I've been very satisfied. It was a small download, easy install and uses about 1/3 of the memory and significantly less processor than Firefox does, especially when you have many tabs open and leave the same instance of the browser running for several days. I've had one crash when setting up Adobe Acrobat preferences, but in general browsing, it is yet to break. Pages load faster, and rendering of HTML and CSS seems fine. The only problem I've had is with Lotus Notes Webmail saying Opera is "not supported" and not wanting to run.
So far, I've installed Opera on older Windows and Mac boxes, and been very happy with the improved browsing experience. I still keep my Firefox handy though, mainly for its great web development plug-ins (Firebug and friends).