Looking at the Browser Global Marketshare Statistics on getclicky.com, there is an obvious 7 day cycle with 5 days high, 2 days low for IE and 5 days low, 2 days high for Firefox both in perfect sync.
My interpretation of this is that people use IE at work but on the weekend at home they use Firefox.
Im guessing it reflects IEs dominance in office Intranets and its integration into Microsofts applications (outlook webmail etc.).
On a side note it looks like Chrome is staying steady on just over 1.5%
James McParlane's Blog 
Interesting analysis, James. Your tweet was right, it is kinda obvious once you think about it. Still, it’s refreshing to know that the browsers stats are generally skewed in favour of what people are forced to used, rather than what they elect to use.
I personally was motivated to leave IE after that virus scare we had a few months past. I suspect that FF greatly benefited from that incident.