I've spent a little while looking at
w3school's browser stats this morning.
IMO:
In 2005, Firefox had the support of the early adopters who looked for a change from IE6. More non-tech-inclined people would have been attracted by the recommendation because IE6 on their machine was probably loaded up with unnecessary toolbars and other junk slowing it down and/or making it unstable. Firefox enjoyed a steady rise in usage for years.
Chrome appears on the scene in 2008 with apparently an immediate 3% market share (which I find a bit surprising). It looks like a good portion of that percentage was nabbed from IE, so my guess is that Google put that link to Google Chrome on the google home page immediately after release.
Firefox hit its peak usage in 2009, and its usage faltered consistently from that point onwards. Chrome's usage steadily went up to the present date, but significantly faster than Firefox's usage ever did in previous years.
My initial thought was perhaps a major reason for this rise was Google recommending Chrome on its home page, but then it occurred to me that Firefox Start was a Google search page hosted on the Google site but did not have a link to Chrome on it. At some later date, Firefox had the firefox start page that was hosted locally.
My next thought was perhaps approximately 5 years is enough for a browser profile to start really going to pot and for the average person to look elsewhere, that and earlier adopters of Google Chrome are probably singing its praises (perhaps for the same reason, they switched from a borked FF/IE profile, just sooner than other people, or some people just like to try something new once in a while), but Chrome has been around for 8 years now. I've seen enough Chrome profiles with crap in them, or the fact that Chrome ate memory for breakfast, which must have put some people off it (or perhaps they upgraded in one way or another?).
I checked when Australis was introduced, it was some time after Firefox began its decline (2013).