What choice did Mozilla have?
It seems like Moz had little choice really. They needed to fix the bug ASAP, and once fixed the source code of the fix would be public anyway, and thus so would the vulnerability.
Mozilla typically fix security issues within a few days of them being discovered. If Opera can't keep up with then, too bad. The choice for Mozilla was either give Opera as long as they need to fix the bug (i.e. wait indefinitely) and leave their own browser vulnerable, or fix and publish.
They made the right decision.