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%