Firefox 3.5 Release Candidate (RC) was the first milestone since Beta 4, which Mozilla released in late April, and the second to carry the "3.5" label. The upgrade was originally called Firefox 3.1, but the company that it had added enough new features to justify the larger bump in number from last summer's Firefox 3.0.

Mozilla has had to delay Firefox 3.5 RC several times because of lingering bugs that defied fixing. Earlier plans had envisioned its release in the first week of June.

In fact, Mozilla took a pair of unusual steps this month as it neared Friday's delivery of Firefox 3.5 RC. It two weeks ago that it described as a nearly-finished RC, but offered the "Preview" only to the 800,000 daily users of Beta 4.

Last week, it repeated the move by pushing another update to the same 800,000.

"As soon as QA performs basic...tests on the milestone, we'll ship the partial updates to our existing beta-channel users," explained Mike Beltzner, the director of Firefox, in a message posted to a on Wednesday. "Then, while our quality assurance team finishes more in-depth functional testing, our large beta audience will be helping us by using the builds across the breadth of the Web. We'll get wider feedback earlier, giving us even more confidence before we declare a milestone as final."