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 only needs to chain two exploits together: One javascript exploit to get local code execution in an isolated sandbox, and this one to jump all the rest of the way to kernel mode.

Which means, you should update both firefox, and your linux kernel.