I was impressed by the amount of periodicity in these two graphs of browser reach, both worldwide:

and for Europe:
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I downloaded the worldwide numbers and cooked them up in a spreadsheet to find that the weekday 68%/26% split between Internet Explorer and Firefox distinctly shifted to something like 66%/27% on weekends. The numbers appear to be a tiny bit more enhanced for the European browser users.
My hypothesis is that the jiggle of a couple percentage points can be attributed to people who are required to use IE on their work computers going home to use Firefox on the weekends.
How geeky is that?