VentureBeat reports Firefox 4 users are urged to upgrade to Firefox 5 because old versions will no longer receive security updates.
When Mozilla launched Firefox 5 yesterday, there were no mentions of security updates for Firefox 4. And for good reason: Mozilla is treating Firefox 5 as 4′s final security update.
Now that Mozilla has put itself on a rapid release development cycle, similar to what Google does with Chrome, old numbered versions of the product will no longer get security updates. Users that don’t update will likely be exposed to vulnerabilities as they are discovered.