I simply don't like FireFox. (I've tried it, I hated it and I deleted it)
Ok back to your problem, are the Opera versions the same? Or is one of them an (old) bčta version? And do you have more then one Opera installations on one PC. If you check all of this, and make sure that you have one Opera installation, and the newest one. And it still doesn't work, try reďnstalling Opera / CCleaner.