A gaping question for me to you Aloone is, on your own day to day browsing of the web are you actually experiencing less rendering issues on Opera than on Firefox? This is the only question that needs to be answered when it come to rendering engine support from the users point of view.
I don't get very many rendering issues with either browser so I haven't notice much difference.
It's just something I noticed when looking at a few the supposedly IE-only sites on the list with Opera. Some of the sites didn't work on Firefox or Opera so I decided to use UA switching (both in Opera and in Firefox, using the extension) to spoof IE. The result of my very unscientific test I noticed that Opera generally did a better job of rendering sites that actively block it. Chrome also did a good job at rendering some IE only sites, although I didn't try UA spoofing because I didn't know how/if it's possible.
Bear in mind that here's a webpage we've forgotten to bring up: the microsuck homepage. edit: guess that applies no more?
I lately discovered that it was a problem for all browsers. The animated gif with fireworks had been replaced with "{ED7BA470-8E54-465E-825C-99712043E01C}" in between me visiting this site with Firefox and Opera.
Firefox doesn't do user agent switching for websites that deliver different content based on UA string, it doesn't have developer-written site specific style sheets saved to work around the problem on a page-by-page basis, it has instead simply become as far as I know, the best at the best for rendering IE hacks over the years in getting to where it is, and if it fails somewhere, click Help > Report a Problem on a misbehaving page.
Opera doesn't do anything clever like automatically switch the UA according to the site, you need to do that yourself. It also has the facility to report broken sites to the developers.
If it's an actual problem for Firefoxes webpage support, I'm sure someone will fix it for the four hundred million or so users*.
One webpage can be wrong.
* Firefox has 24.72% market share according to Wikipedia, and there are 1.7 billion internet users according to http://www.internetworldstats.com/stats.htm
For a start there are many different measures of browser market share and since when did that make a difference to quality? Internet explorer has over four times Firefox's market share, it must be better. :
I've just got Opera installed on Linux, it looks pretty good, not much different than it did under Windows. I've still got to get round to setting up adblock but when I do I'll start using it as my main browser.
EDIT:
You know the rendering error I was experiencing with Firfox under Linux and Opera under Windows?
It doesn't occur with Opera under Linux.
It's funny how changing the OS reverses the situation. One might have though that an OSS browser would work better under an OSS OS. ;D