Opera Software has announced that they would support OS X Sandbox with their next major update Opera 12.
The company said:
Opera was the first major web browser in the Mac App Store, and we are making sure we stay there by introducing support for OS X Sandboxing in Opera 12. Sandboxing is a operating system level security measure that limits an applications access to the system to prevent malicious code from taking over an app and causing damage to the system. OS X Mountain Lion also introduces Gatekeeper, protecting users by only allowing Apple developer signed or sandboxed applications run on the system. Opera will be one of these applications. Look forward to even more of OS X Mountain Lion’s new features in the future.
They have released a build of Opera 12 for Mac featuring these features for testing.