The Firefox 3/Gecko 1.9 development team has decided there needs to be a Beta 5 release of Firefox 3.
…based on the number of blockers remaining. This additional beta will ensure that changes which may affect website compatibility and changes which affect the user experience will get exposure to a wider audience for feedback and regression testing.
The string freeze is expect this Friday, March 7th with a code freeze on March 18th. They say Beta 5 will be the last milestone for string changes. No word as of yet on a release date for Beta 5, especially given Beta 4 hasn’t even been released yet.
Source: Mozilla Developer News