Firefox 3.7 / Gecko 1.9.3 Alpha 3 Cooking.
Mozilla is hard at work on the next major version of its open source browser, and early adopters can already get a taste of the successor of Firefox 3.6. Two preview releases of Firefox vNext have already been made available for download, and could very well be followed by a third by the end of this week. Mozilla's Benjamin Smedberg reveals that plans are in place to deliver a third early preview of Gecko 1.9.3 to testers at the end of this week.
Source - Firefox 3.7 / Gecko 1.9.3 Alpha 3 Cooking - Could be released by the end of this week - Softpedia