I tend to think of rootkits as having kernel-mode black magic to be super-stealthy, including across reboots.
And in one memorable case, across a full Windows reinstall. (IIRC, something about a custom boot sector and some code hidden in the 1MB of a disk that Windows reserves for their GUID partitioning scheme. As I say, black magic.)
As such, I wouldn't class a browser plugin, however stealthy, as a rootkit. But obviously some people do:
The plug-in can be classed as a rootkit because it is thought to allow continued privileged access to a machine without a user’s consent. [1]
And Wikipedia's definition could be twisted to fit:
A rootkit is a collection of computer software, typically malicious, designed to enable access to a computer or areas of its software that would not otherwise be allowed (for example, to an unauthorized user) while at the same time masking its existence or the existence of other software. [2]
So potayto, potahto. It's all in the definition you choose.
Can we all agree that that a plugin allowing any website to execute arbitrary software with arbitrary command-line parameters was a massive security problem? Then we can move on.
So what about that Train Fever on the current humble weekly bundle? It looks like you can build track but not drive the trains, is that correct?