Mozilla have made available for testing Firefox 3.6 release candidate 1
This is pre-release software, if you use it you may encounter compatibility problems with some websites and add-ons. A release candidate is a version of the software that has the potential to become the final release version, ready to release unless serious bugs are found. If you do test the release candidate and find any bugs, you can report them to Mozilla using the mozilla.org feedback system, or if you have a specific and detailed bug report you can use Bugzilla, the Mozilla bug reporting system (you will need to register an account at bugzilla)
For more details see Mozilla Firefox 3.6 Release Candidate Release Notes
For those that use the IE Tab extension, it does not work with Firefox 3.6, and development of that add-on appears to have been discontinued. There is an alternative add-on that should work, Coral IE Tab.