The SO (Sheriff's Office) was clearly able to do at least a cursory read of Firefox v2 data early on in the case, because that is how they discovered the Google searches for chloroform in September of 2008. So even if their tools were broken with respect to time stamps, visit counts, and other details, they worked well enough to tell them what sites were visited on what dates.
If they had run those files even through their flawed tools, they would have seen that someone did a Google search for fool-proof suffocation on June 16. They would also have seen that the same someone was very active that day on social media.
At the end of the day it seems that the investigative protocol at OCSO did not involve timeline reconstruction. I know we at Websleuths devoted a great amount of time trying to reconstruct the timeline for June 16 and July 15 / 16, as well as June 17, 18, 19, and 20. We also worked to establish behavior patterns prior to June 16 to see if, for example, the "flurry of phone calls" on late afternoon June 16 was unusual or not (it was not).
As a layman, all I can say is that it seems there was a lack of curiosity on the part of the SO as to what Casey was doing on the days from June 16 to July 16. They did not seem to try to marry the cell phone, computer, and interview record together in a cohesive manner.