Favicon not updating; flashes new version, then reverts

I'm trying to update the favicon on my site, but something strange is happening: it shows the new version for a split-second, then reverts to the old icon. Check out yesinsights.com to see what I mean -- watch the favicon carefully. Here's what the favicon should look like.

I've tried adding a query parameter to the favicon url, I've tried clearing my browser's cache (I'm using Firefox on Ubuntu), I've tried all kinds of variations on the <link> tag, and nothing seems to work. Any help would be very much appreciated. Thanks in advance!