Do not allow VP9 publishing on FF (#894)
* Do not allow VP9 publishing on FF
Since FF does not support SVC, and we require SVC for VP9, it will not
be able to support VP9 publishing.
Previously FF didn't support `getCapabilities`, so supportsVP9 check
was passing for it. As much as I hate browser checks, there isn't
another way to check for SVC publishing.
* changeset
