The only concern I share with the author is this:

The isolation provided by Docker is not as robust as the segregation established by hypervisors for virtual machines.

As seen with CVE-2015-3629[0] for instance.

The other points: patch level and docker management isn't understood, seem to be people problems which can easily be corrected.

[0]: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-3629

So Docker running your firefox probably won't be as secure as doing it in a vm, but it will start pretty much instantly on your desktop where your vm wont, and it will be more secure than just running it natively.