Officially released on March 22, Firefox 4.0 has been a success for Mozilla with hundreds of millions of downloads worldwide as seen on the official Glow page, but the transition process from older versions to the latest one hasn't been as smooth as the Mozilla team hoped. As noted by 9to5mac, users still running the old 3.5 version of the browser have become a real problem for the company, which is struggling to find a way to EOL (end-of-life) the product by somehow forcing or promoting a recommended update to Firefox 4.0 for desktop machines. With around 12 million users still running Firefox 3.5, this means Mozilla has to take care of security and stability updates for an old version, and on the other hand users (who might still be unaware of the update or simply don't have the permissions to run a software update on an office computer) can't enjoy the latest features and additions to the browser. Either by generating buzz on blogs and tech websites about the new release or simply displaying a popup on screen asking to opt-in for the update, Mozilla is moving forward on its plan to kill the old 3.5 version.
We need a plan to obsolete Firefox 3.5 as we can't support it into perpetuity. We have been frustrated with our efforts to move users off of old releases and are worried too many people do not upgrade and are on vulnerable and unsupported versions of Firefox.
In the meantime, things are quickly evolving on the Firefox 5 side. Mozilla is adopting a release cycle similar to Google Chrome, with major updates coming out every few months instead of years. After launching Aurora, a channel for releases between "nightly" and "beta" status to give users a rather stable glimpse of new features to come in Firefox, Mozilla has announced the milestone update will come out on June 21st, with the 5.0 version moving to the beta channel today. Firefox 5.0 is still available in the Aurora channel at the moment of writing this, and we expect the beta to officially go live later today. Among several stability improvements and performance enhancements, Firefox 5.0 will let users "pin" websites, with the browser generating app-specific links for websites like Facebook. The update system for add-ons will go under a major update, and the address bar is also rumored to receive new social sharing functionalities.