Release candidate builds are supposed to be less buggy than beta builds but may have a few lingering compatibility and stability issues.
Firefox 3.6, an update from the current stable version 3.5.7, was suppose to be made available in Q4 2009, but Mozilla pushed back its target release date last month.
The next major Firefox release, version 4.0, was also pushed back until late 2010 or early 2011, with a beta due this summer.
Firefox 4.0 includes a project called Electrolysis, which will launch each tab window under a separate process, an innovation that first appeared in Google Chrome. This will help make Firefox more secure and more stable. Firefox 4.0 may also bring user interface changes.
Mozilla faces a rising challenge from Google Chrome, which by NetApplications' measure last month passed Apple's Safari browser in global market share. That challenge is magnified by the increasing importance of Web browsing on mobile devices, where Firefox is not as well-established as it is on the desktop and is unavailable on devices like the iPhone.
Nonetheless, Firefox's growth appears to be healthy. "In the 4 months leading up to the holiday season, Firefox added 22.8 million active daily users," said Mozilla's "analytics guru" Blake Cutler in a blog post on Friday. "During that same period last year, Firefox added 16.4 million users."
The official release of Firefox 3.6 should happen before the end of the month, assuming no show-stopping bugs are found in the release candidate.
Version 3.6 allows users to change the way Firefox looks, thanks to built-in support for Personas, a Mozilla project that allows easy customization of the browser's appearance.
Firefox 3.6 will no longer load third-party components installed in its internal components directory, a change that promotes stability and security. It also introduces support for a number of new features for Web developers, such as the ability to detect device orientation and accelerometer support in Mac laptops.