Although, does Firefox not use separate threads for each tab? If so, then it would seem like in normal operation, it should be possible to achieve parallelism.

Although, does Firefox not use separate threads for each tab? If so, then it would seem like in normal operation, it should be possible to achieve parallelism. Of course, I do still see whole-browser lockups in Firefox, and what I really miss is the ability to actually diagnose misbehaving tabs. Also, I think the Chrome debugger has surpassed Firefox's, especially with the experimental stack traces in asynchronous flows of control.