Earlier quoted context omitted.
Not really. Generally we use "Local Privilege Exploit" to describe an exploit that goes from a reasonably normal user privileges to root privileges. And we don't usually worry about them, because an application with normal user privileges can already to so much damage. But this exploit can be triggered from inside a tightly sandboxed process, such as firefox's isolated browser process. Which means the attacker now on…
Realistically, if you have a browser sandbox, the system LPE exploit gives you very little more. Everything interesting on a desktop system is accessible by the user account directly.
This gets you code execution in the kernel, at which point sandboxing is irrelevant.