Beta 10 was just released and Beta 11 is due early next week. With some luck, we should see the release candidate within the first two weeks of February.
However, the next version of Firefox is unlikely to require almost a year to be completed. Mozilla's CTO, Brendan Eich, recently said that Firefox 4 will be released within four month after the final release of Firefox 4. This seems to be a result of discussions at Mozilla to accelerate the release schedule, which is one of the reasons why Google is gaining market share. Faster release cycles support the perception of a fresher browser, whether it delivers lots of new features or not. If you look at it, Firefox 3.6 looks pretty antiquated today.
The next few weeks should be interesting as Microsoft is also putting its finishing touches on Internet Explorer 9. IE9 RC is rumored to be released on Friday, January 28.