Previously developed as Firefox 3.1, Firefox 3.5 Beta 4 is expected to go into a code freeze soon. Adding enhancements to the TraceMonkey Javascript Engine, 3.5 Beta 4 should be available in the coming week, along side Firefox 3.0.9, which will be a regular maintenance update.
At the current rate, it is expected that Mozilla release the final version of Firefox 3.5 just one year after the 3.0 release.