On a personal note, I've been using Firefox 3.6 beta 1 as my main browser for a week, the startup and overall speed boost over 3.5.x is impressive. There are still a number of add-ons that don't work (officially) with Firefox 3.6. To that end, Mozilla is recommending that beta users install the Add-on Compatibility Reporter in order to help add-on developers get their tools up to speed.

For those that have no patience (like me), for some add-ons the process of making an incompatible add-on work with Firefox 3.6 can often be quite simple. Add-ons include a line in their code that identify a max version number of Firefox with which the add-on will work. By changing the max version to Firefox 3.6 (or even better 3.7), it is possible to get some add-ons to work, prior to the official developers committing a 3.6 version of their add-on.

It's not clear to me the exact date of when Firefox 3.6 will be made generally available. It's always a matter of making sure there are no blocker bugs prior to a release. At the current rate though I'd certainly expect to see the final Firefox 3.6 out before the end of the year, if not by the end of this month.