Firefox will still be releasing a new testing version, but it will still be considered an Alpha, as opposed to moving to Beta, as was originally planned.   Making a solid browser comes before meeting imposed deadlines, and the big hurdle is the upgrade to the Gecko engine to 1.9 to allow Firefox3 to pass the Acid2 test.  You can see the updated roadmap, and a bit more detail on what is going on at Ars Technica.
“As we have previously reported, the first official Firefox 3 beta release was tentatively scheduled for late July. The beta has now been pushed back due to performance regressions and the need for extra front-end development time, and the roadmap has been altered to reflect a new release plan. According to Mike Schroepfer, Mozilla’s vice president of engineering, the developers intend to issue a new pre-beta milestone release every six weeks until the program achieves beta status.”

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