Firefox and testing concurrent web application
TL;DR: firefox won't start a request for an URL if a request is already being executed for that same URL. If you open http://example.com/foo, and it takes several seconds to complete, when you simultaneously open a new tab and enter the very same URL, the latter request WILL NOT BE SENT until the former has completed. If you ever need to make parallel requests for the same URL make sure you put some random query string in them, so the browser won't serialize requests for them. I was testing a concurrent web application written in Haskell, Yesod. I love Haskell and the Yesod framework, this combination gave me the best web development experience so far. I especially loved how easy it was to develop a simple web-based chat application by using TVars and TChans - variables and "channels" - basically message queues - to store state in a Yesod web application and pass messages between threads. I stumbled into a strange bug, and it took a good two hours of my life...