Mozilla issued the first such build last Tuesday, calling it "rc1build2," said Beltzner, because it was "the first revision of the release candidate for which we'll ship updates, and it required two build attempts,"

The new process is somewhat akin to what Google has been using with its own Chrome browser. Google, for instance, maintains a -- developer, beta and stable -- each updated automatically as new builds are available, each channel more stable, and thus offering older versions, than the one before. Although Mozilla also uses a similar multi-channel approach, this is the first time it has offered incremental, partially-tested code to large numbers of users.

Beltzner said that the final version's target date remains the end of this month, although to ship Firefox 3.5 by then will require a faster pace than last year's delivery of Firefox 3.0. If Mozilla decides to roll out a second release candidate, it will likely miss its deadline.

Firefox currently accounts for , according to Web metrics company Net Applications' May data. It faces renewed competition on almost every front, including Microsoft with its Internet Explorer 8, Google's Chrome browser, Opera Software's Opera and Apple's Safari 4, which launched earlier this month.

Users running Firefox 3.5 Beta 4 or one of the incremental release candidate builds should receive an offer to update to the Firefox 3.5 RC. They can also choose "Check for Updates" under the "Help" menu to do a manual update.