Mozilla surprised us a bit with an unexpected release of the first release candidate for Firefox 3 last weekend. It's a likely sign that the official release is nigh.
When will that be? You already know the answer: "When it's ready." My best guess is sometime in the late afternoon, either this month or next.
That said, Release Candidate 1 is going to be close enough to the final version of Firefox 3 that you won't notice any significant differences...until of course, you check your add-ons--extensions and themes. A large number might be incompatible with RC1.
However, let's cut the developers some slack. If an extension is compatible with Firefox 3 beta 5, it will be compatible with RC1 shortly. There's some frantic updating that's likely happening right now. However, if you like Tab Mix Plus a lot (which I do), you might be waiting a bit.