User-Agent: Opera/9.50 (X11; Linux i686; U; en) Build Identifier: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.14) Gecko/20080419 Ubuntu/8.04 (hardy) Firefox/2.0.0.14 Both 2.0.0.14 on linux and 3.0 on windows have same problem. Enter string "javascript:void(setInterval(alert, 3000))" in address bar and press <Enter> or "Go" button. This will call alert function every 3 seconds. Function called will receive random numeric value as a parameter in firefox, while other major browsers do not pass any parameters to this function. This quirk is not documented here: http://developer.mozilla.org/en/docs/DOM:window.setInterval Reproducible: Always Steps to Reproduce: 1. Open any page in firefox. 2. Enter string "javascript:void(setInterval(alert, 3000))" in address bar and press <Enter> or "Go" button Actual Results: Alert dialogs appear one after one, displaying random numbers. Expected Results: Alert dialogs should display 'undefined' or empty string.

see bug 394769 comment #2 Duplicate of bug 394769 ... or Bug 10637

Looks like that