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.