Sometimes bugs are more interesting to work on than features - they have that mysterious quality about them and give a satisfying feeling when you figure it out.
This one was brought to my attention by Mark Finkle.
It basically boiled down to HTTP POSTs from Firefox on Windows being slower than they are in Internet Explorer, and also slower than they are in Firefox on OS X or Linux. (IE on windows and the non-windows platforms all perform about the same, with FF on windows lagging behind).
The culprit turned out to be the TCP congestion window. Firefox never had more than 8KB of un-acked data outstanding. If you have a network path with a high bandwidth-delay product, that isn't going to cut it.
Windows (up to and including Vista) has an 8KB default sending window. Or so I found out thanks to Google.
Autotuning that buffer size is standard practice on OS X and Linux and has been for a long while. Vista autotunes the receive buffer (but not XP according to what I read), but the send buffer is a small fixed value. IE, realizing that its a web 2.0 kinda world out there full of User Generated Content, must increase that value from its default - because I can look at the IE tcpdump traces and see >80KB of un-acknowledged data (there would be more, but the max window size is not the limit at whatever value they have it set to) in the same way I do with a trace of Firefox on Linux.
The Linux default is 128KB for any reasonably modern machine.
Fortunately that can be controlled on a system wide basis through a registry preference, or on a per socket basis by setting SO_SNDBUF. I submitted a patch that does the latter if the network.tcp.sendbuffer preference is set - the patch also sets the pref for windows.
If you would suffer from this, I see three options:
1] Wait until my patch (or a later rev of it) ends up in an official build
2] Set the registry property to change it for your whole Windows install - KB 950326.
3] you might be able to build a k3wl binary add-on that does the same thing as my patch in a crazy way. Fame, fortune, and faster flickr and picasa uploads await you.