One of the plugins I mentioned seems to be delaying the creation of new firefox windows by about 12 seconds. New tabs (ctrl-click) open instantly.
It's not on account of any network traffic, either. There are no web (or other network) requests being sent immediately, then timing out after 12 seconds.
I think it's Remove it Permanently. I've replaced it permanently with greasemonkey, and new windows now open within a few seconds.
Argh!