I am big fan of Fluid and use it for creating apps for most of my email/social sites. I have also one created for Campfire which I use to communicate with my team. Recently my Campfire app stopped working (not sure if this happened after I upgraded to newer Fluid version or whether the Campfire dev started using a single signon servide ) in the sense then when I click on 'Signin' it was opening my default browser and forwarding me to "https://launchpad.37signals.com/signin". On further googling found that this was happening because the campfire fluid app did not have permission to allow other site domains meaning the signin process was forwarding to a different domain than the campfire specific domain. For instance my campfire domain is "https://icrossing.campfirenow.com" and signin would redirect to "https://launchpad.37signals.com/signin" for authentication. To make it work all it needed was to set the app permission to allow browsing to other domains and that made it work.
Go to fluid app -> preferences -> Advanced and enable the below option