User Agent: Mozilla/5.0 (X11; Linux i686; rv:17.0) Gecko/20100101 Firefox/17.0 (Beta/Release) Build ID: 20130805152501 Steps to reproduce: From version 24, if I try to fire an alert in a dialog window I get the error "cannot call openModalWindow on a hidden window".

Try: 1) safe mode: https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode 2) new profile: https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles Does it fix it?

No, it doesn't. I found out that it happens just in onload procedure (through onload attribute or addEventListener), so it's the same of bug https://bugzilla.mozilla.org/show_bug.cgi?id=912655, but I don't understand why it's closed as invalid. The bug is present and in my opinion the alert should be fired in onload procedure too. If this intentional (and I hope not, because it's very annoying in debug operations), the error message is not correct: at least it should say that the window is loaded but not visible yet! A "hidden window" seems meaning something different, a window that - for its nature - is never visible.