From the article, the usual way that Firefox updates is when you open the browser Mozilla detects that you're online and insists that you install any available updates before you can begin browsing.
(Note that I don't really know if that's how it works, but that's what the article implies.)
There has apparently been a lot of complaining about the delays when updates are available (usually once every 6 weeks).
The article implies that beginning soon for the latest version, and shortly later for all versions, updates will download to your machine (with no notice to you). It's quite likely that the download can take place as a "background process" invisible to you.
The article isn't clear about when/how the actual installation of the update will take place, but if the intent is that use of the browser won't be affected, it might be assumed that the actual installation could take place when you close the browser (???).
The INTENT, stated in the article, is to assure that everyone uses ONLY the latest version of the browser and that the latest updates are installed promptly. This would appear to preclude using an older version, since the automatic silent upgrade would (eventually) replace any old versions.
If you've installed version 1.5 or 3.6, your installation will be upgraded to the current one (I think I saw ver 12 or 13 is current now?), if the article means what it says.
This may not really be what they meant, and those more familiar with how Mozilla does its business may be able to make a better interpretation of what the Mozilla übermeisters are saying, but that's the way I read it. (It's also possible that the writer of the article may have "slightly misquoted." My respect for tech writers doesn't necessarily extend to an "automatic assumption of absolute accuracy.")