While the trend is correct, I would argue that the original authors statement that this is due to Chrome having been "a faster, more secure and versatile browser" is omitting the elephant in the room:

The main reason for Firefox's (and IE's) decline IMHO is the pink line "Mobile vs Desktop" in the chart you linked to. People won't bother installing a different browser if the one that comes with the device they use is halfway decent (on iOS, they don't even have a choice). And so as more and more browsing comes from smartphones and tablets, more and more people will use Chrome (Android) or Safari (iOS).

Unfortunately, this is quite a depressing finding for Firefox, because it means that all technical enhancements will not be able to move the needle much. (The rise of Firefox on the desktop was arguably helped by the EU requiring Microsoft to implement the browser choice window in Windows; unless something like this comes along for the dominant mobile platform(s) it will be an uphill struggle.)

Edit: And the move to mobile might even affect desktop browser usage because of things like link- and password-syncing. If you use Chrome on mobile, you might want to also install Chrome on your PC to be able to sync your settings across.