According to the data, Firefox 3.6 still commands a 5.92% usage share, putting it at position number 5, behind IE 8 (24.23%), Chrome 15 (21.93%), Firefox 7 (14.68%) and IE 9 (9.95%). Firefox 6 usage by comparison has fallen to 2.35%. Considering that Mozilla released Firefox 4.0 more than six months ago (March 22, 2011), it's holding its ground incredibly well.
Firefox 3.6 was the last 'traditional' release of the browser before Mozilla switched to a rapid release cycle that sees a new version making an appearance every 6 weeks. In the home environment, this is not a big problem. Sure, some add-ons might break, and people might have to spend a few minutes scratching their heads trying to find things that have moved, but none of this is a deal breaker. For business users, this aggressive release cycle is too much to have to deal with (because enterprise likes to test things before releasing them). It's simply too much to ask enterprise users to to test a new browser every six weeks, especially when there isn't much to appeal to enterprise users in those releases. Given this, sticking with version 3.6 makes a lot of sense. Mozilla has outlined plans to offer enterprise users a special versionESR (Extended Support Release) of Firefox that would have a more relaxed 42-week update cycle, but so far no ESR browser has materialized.
Firefox 3.6 is Mozilla's Windows XP ... that particular version that people love and will cling on to for dear life. And people are clinging onto Firefox 3.6 for the same reasons that they are clinging onto XP. First reason, there's no compelling feature or reason driving people to move on. Secondly, people don't like UI changes for the sake of UI changes. Some UI changes make sense, but others seem to be done of a whim based on what developers wanted to see and not what users wanted. Thirdly, people aren't upgrading because they don't have to. There's no carrot and there's no stick.
Mozilla needs to fix the situation it's found itself in. It can do this by doing these two things:
Releasing an ESR version of Firefox
Announce an EOL (End of Life) date for Firefox 3.6
Doing this would send a clear sign to users that it's time to upgrade as opposed to the confusing message that Mozilla is currently sending out ('You should upgrade ... but here's an update for that old version you're running.').