http://www.theregister.co.uk/2011/05/16/mozilla_firefox_3_5_forced_upgrade/
Mozilla plans to issue a warning to all such users telling them they are running an out-of-date browser that is no longer supported against the threat of online attacks.
The message will be displayed through Google's default search page starting tomorrow.
.21 June, the same day Firefox 5 is expected to be released.