User Agent: Mozilla/5.0 (X11; Linux i686; rv:15.0) Gecko/15.0 Firefox/15.0a1 Build ID: 20120511030513 Steps to reproduce: Allowed Nightly to update itself. Actual results: The first attempt to invoke the "Check For Updates" behavior via the Help->AboutNightly dialog always fails (times out, fails to connect) on the first attempt and always succeeds (connects) on the second try. Expected results: One attempt should be enough, assuming network and server are functioning.
Have you got NoScript installed? I see this too, but only when NoScript is enabled.
It should be fixed by NoScript 2.4.2rc2 from http://noscript.net/getit#devel -- thank you for reporting.