While Google's Chrome is blazing through version numbers, Firefox 4 has a drawn out roadmap ahead of it. On December 9 2010, Firefox released 3.6.13 update, with some security and stability fixes.
Firefox 4 Beta has been out from June 24 2010, with new features being added in every beta iteration till Beta 7. The beta 7 feature set will then be refined through beta 8 to beta 10. Beta 10 will be the last beta release and is expected in January 2011. The Firefox 4 RC is expected in early 2011.
Mozilla expects to give Firefox 4 a run of around 6 months before the final release candidate, exposing the browser to around 1M to 4M users. This should polish rough edges.
Using Firefox 3.6.12? http://getfirefox.com should give you the stable version.
Firefox 4 beta is available here as of now