On the road to the first Release Candidate build of Firefox 3.5, Mozilla has made available for download a Preview development milestone. The RC iteration of codename Shiretoko, previously Firefox version 3.1, will continue to cook even after the availability of the Preview version, which is designed for the existing users of Firefox 3.5 Beta. In fact, Mozilla plans to release Firefox 3.5 Preview, also labeled Beta 99, as an update to existing testers that are running the last Beta development milestone of the open-source browser.

Mike Beltzner, director of Firefox at Mozilla, explained that the Preview Build of Firefox 3.5 is designed to allow developers to perform further testing of the open-source browser, and for the community to generate additional feedback for the RC stage. At the same time, Beltzner promised that the fully fledged Release Candidate of Firefox 3.5 was coming soon.

“The 800,000+ users of Firefox 3.5 Beta 4 will be receiving an update to a “Firefox 3.5 Preview” release in order to assist us with further testing and feedback. This update contains many fixes for stability bugs, correctness fixes for our JavaScript engine, and improvements to our built-in support for open Ogg based video and audio playback. While this release has not yet completed the testing required to be declared a release candidate, it is considered stable for daily browsing use and we appreciate your assistance in helping us test and evaluate this preview release,” Beltzner explained.

Users already running Firefox 3.5 Beta 4 should start noticing the Preview build being offered as an update in the next 24 hours. Mozilla will be pushing the new release as an automatic refresh. Alternatively, Firefox 3.5 Beta 4 users can check for updates via the Help option in the menu, or download and install the bits manually.

“If you’re not yet a Firefox 3.5 Beta user, we recommend that you wait for the upcoming Firefox 3.5 Release Candidate, which should be in the next 1-2 weeks,” Beltzner added.

Firefox 3.5 Preview (Beta 99) is available for download via this link.