It seems Mozilla has decided that Firefox Version 4.0* was simply a minor detour in their browser roadmap and it’s time to move on to version 5.0. In short, there will not be any more versions of Firefox 4.0*. Your customers should have received a notification around June 24 that ‘strongly recommends’ they upgrade to version 5.0.
Version 5.0 can be downloaded from the following URL – http://en-us.www.mozilla.com/en-US/firefox/new/
NOTE: I will also mention that vulnerabilities have been identified in all versions of 3.6 prior to 3.6.18. For that reason, it is recommended that most individuals go straight to version 5.0