Can you or anyone else really back that up? Opera seems very stable and accurate at rendering for me.
All programs can crash and fail to render some things properly - Mozilla (and therefore Firefox) is no different. I used to view a lot of images with Mozilla, and after so long it would always crash, losing my current page. The problem never occurs with Opera (where at least I can resume from the same page after any crashes).
I have also heard of problems with Firefox extensions conflicting with each other, causing headaches. Opera gives you peace of mind as there is nothing more to download. Sure, Firefox will offer many extensions for things Opera doesn't include, but many are things that should have been included to start with. (One is even called something like "Things they left out"!) And who wants to download dozens extensions on a 56K modem?