I assume its becuase they had no chance on competing with Firefox while it wasn't free, (the adds were annoying). Opera has lots of very useful features which are very handy for web developers like myself.
Firefox has a major advantage on thats its open source, if there is a bug it can be fixed very quickily in Firefox, like IE Opera is commerical software so bugs can't be fixed as quickly.
If a website is coded properly there should be no issues regarding its display on any browser.
So which is the better browser? Opera is technicaly a lot better, it has more features, its faster etc etc but Firefox is so simple and as a major advantage in that its open source.
One thing with Opera though make sure you change the settings so that the page updates on every visit, if you don't dynamic sites such as this forum will appear static (i.e it will appear that no new threads have been made).