Mozilla has released a fifth Firefox 4.0 development preview , labelled Firefox 3.7 Alpha 5, based on the latest Gecko web rendering platform. This alpha adds support for the WebM video format, and there are 64-bit builds for Linux and Mac OS X

Mozilla has released third Firefox 4.0 preview, labelled "3.7 Alpha 3", a developer preview of its latest Gecko web rendering platform build

Firefox 3.6 is scheduled for release as a stable version in November 2009, so this release is just a preview, the starting point for development of Firefox 3.6.

"Please note: Firefox 3 Beta 5 is a public preview release intended for developer testing and community feedback. It includes new features as well as dramatic improvements to performance, memory usage and speed. We recommend that you read the release notes and known issues before installing this beta..." -- ftp://ftp.mozilla.org/pub/firefox/releases/3.0b5/linux-i686/

A Mozilla Developer Preview of improvements in the Gecko layout engine is now available for download. This is a pre-release version of the Gecko 1.9.3 platform, which forms the core of rich Internet applications such as Firefox.

Previews of Firefox 3.7, which will in time become 4.0, are now available. The Firefox developers have released a new preview version of Firefox 3.7 which includes many of the features planned for Firefox 4.0. The preview release includes special protection against rogue third-party processes that could crash the browser.

A Mozilla developer preview of Gecko 1.9.3 has been released. The release comes in the form of a nightly build of Firefox 3.7 alpha 3. The new build introduces several new features.

"Please note: This Firefox 3.5 Preview is intended for developer testing and community feedback. We recommend that most users wait for the official Firefox 3.5 release, which is coming soon..."

KDE released the first alpha of Frameworks 5, part of a series of releases leading up to the final version planned for June 2014. This release includes progress since the Frameworks 5 Tech Preview in the beginning of this year.