It should be noted that while Firefox doesn't have tab sandboxing, it does isolate Flash (by launching it in a separate process called plugin-container).
Here's how firefox describes plugin-container [0]:
Plugins are loaded separately from Firefox, allowing the main Firefox process to stay open if a plugin crashes.
This is different than a sandbox. A sandbox limits what the plugin is capable of doing to the system. The whole point of sandboxing flash is that it has had numerous security holes.
