I read that article as explaining why a web application can't do crypto with javascript. As someone that knows almost nothing about browser extensions, can you elaborate on why one isn't a good idea for chrome?
The sections "How are browsers hostile to cryptography?", "What systems programming functionality does Javascript lack?", "What else is the Javascript runtime lacking for crypto implementors" cover issues you would encounter with browser extension cryto.
Ah yeah. I believe the usual terminology is such that "extensions" are javascript and "addons" are something native. You could probably do cryto well with an addon.. to the extent that it is possible to do an addon at all properly (honestly I have no idea there).