Opera has a very optimized rendering structure. it is capable of drawing the page element by element without needing to wait for the entire structure.
I think the reasons so many prefer Firefox are rational enough, rather than pseudo-religious. The clear interface, the ease of use of the tabs, as well as the wide range of plug-ins (largely a result of its open source origins) all add up to an excellent browser.
However, Opera is great browser too. The voice control features and the amazing range of views achievable make it a must for widening access to the Web, while the Transfers page is a really excellent tool for contolling multiple downloads. It's also got an integrated email client.
The problem is that most users seem to think they must commit themselves to one browser. The rational course is to install a range and use them for their specific strengths. I use both Firefox and Opera as I see fit.
In computing, recommendations often have a pseudo-religious nature. Firefox was claimed to be innovative for adding features that Opera already had because Firefox is open source and free. Opera's genuine innovations, superior accessibility (eg for blind users), robustness, standards-compliance and security were ignored because it is a commercial program.
Also, Firefox comes from California, whereas Opera comes from Norway. However, Opera has found support from companies based outside Silicon Valley, including Nintendo, Nokia, Sony, Sharp, Archos and Psion.