After a little more than a year from Firefox 4's release on March 22nd, 2011, Firefox 3.6 will cease to be updated anymore. Some users still hold on to this version in protest of Mozilla's interface changes and rapid release cycles as well as extension breakages. While the "interface changes" issue is highly subjective and can be resolved in one minute with a theme, the other two issues are becoming less and less relevant as time goes by.
The problem was that before 4.0, add-on developers expected major Firefox releases every one or two years and planned their development accordingly, tying their add-ons to specific versions. When major releases started coming every 6 weeks, there were a few months when a lot of extensions broke. Mozilla was reasonably quick to step in and create an internal tool that would automatically analyze extensions and mark them as compatible with newer versions if that would be the case.
Additionally, Mozilla has released the Add-on builder and the Add-on SDK to help developers create restartless extensions that would work with any new Firefox version without issues.
This release caters to enterprise users and users adverse to change in general. Starting with version 10, Mozilla will support a version for 9 releases (54 weeks). This means that while that version doesn't get any new features, it will get the latest bug fixes and security patches. Every 7 releases, a new ESR version will be created, so the next one will be v17. Two successive ESR releases will overlap for 12 weeks, allowing users to test and upgrade their systems to the newer version.
Community response has been mixed. While most welcome the ESR, some say that it's still not enough. Corporate users usually expect a product to be supported for 3 to 5 years because that's how long the hardware is expected to last and don't want to bother to upgrade the software sooner.
While this is understandable from a cost perspective, it's why web developers had to struggle for the last years to offer support for Internet Explorer 6 (a browser from 2001). This need for support held back development and adoption of modern web technologies for much too long.