Mozilla has rolled out the release candidate for Firefox 3.0, bringing the open-source browser one step closer to its first major overhaul in nearly 19 months.

The appearance of Firefox Release Candidate 1 (RC1) came earlier than expected. As recently as last Saturday, Mozilla's chief engineer said that although the company had locked down RC1's code, it was planning to publicly launch the build in "late May."

Firefox RC1 is available now for download from Mozilla's servers, and is also being offered as an update to users running Firefox Beta 5, the final beta edition that shipped six weeks ago.

Even so, Mike Beltzner, Mozilla's interface designer, voiced the usual warning to ward off casual users. "The Firefox 3 Release Candidate is a public preview release intended for developer testing and community feedback," he said in a post Friday evening to Mozilla's developer centre blog.

Beltzner touted user interface changes, stability and compatibility fixes, and additional performance improvements made to RC1 since the last beta. The RC1 release notes, meanwhile, cautioned users that some add-ons may not work with the candidate.

Although Mozilla has run through multiple release candidates in the past - three before moving on to final for Firefox 2.0 in last 2006 - Mike Schroepfer, vice president of engineering, said a week ago that RC1 might be the sole build prior to calling the browser done.

Schroepfer has said several times that Firefox 3 will ship in June.

Firefox currently accounts for about 17.7 percent of the browser market, according to the most recent data from Net Applications. Microsoft's Internet Explorer remains the most wide-used browser, with a 74.8 percent share, while Apple's Safari comes in third with 5.8 percent.

Firefox 3 RC1 can be downloaded for Windows, Mac OS X and Linux in 41 languages from Mozilla's website.