I've been assuming it's someone's "clever" JS snippet doing the rounds, perhaps trying to normalise behaviour on some mobile browser or something along those lines. However, looking for obvious things like an onscroll or onmousewheel event handler attached to any element in the DOM turns up nothing, so whatever technique they're using to do it is new to me and I don't know how to turn it off. :-(
I'm running release channel Firefox on Windows 7 if anyone's curious. A range of other pages I have open in different tabs right now are all scrolling at my normal (much faster) speed.
I don't think we have any scroll event handlers on our blog (not 100% sure as I didn't implement the theme). It's a WP site, perhaps that's the common element -- although I don't think WP ships any JS itself...
My best guess would perhaps it's animated gifs triggering a bug in the browser for some reason?
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EDIT: animated gifs more likely.
That's plausible. Firefox has been buggy as an insect colony in recent releases, and other browsers didn't seem to have any problem with it.
In case it's useful to know: the page was also very slow to zoom at first in Firefox but then when I tried again a minute or two later it seemed to be back to normal. Again, this was a very obvious difference.