From the 'don't count your chickens till they hatch' files:

Firefox 3.0 is not yet officially out the door but Mozilla developers already have a plan -- and a timeline - for Firefox 3.1. The draft plan -- as it is currently being floated on the Mozilla developer lists -- is for Firefox 3.1 to be released within six months of the release of Firefox 3. Considering that the final Firefox 3 is likely within a matter of weeks that would mean we'll see Firefox 3.1 in 2008.

The release of Firefox 3.1 will also mark the likely end of life for support of the current Firefox 2.x browsers. The plan (as it has been since the release of Firefox 1.5) is to have security/stability updates once every two months or so, until the next 'feature' release -- which is Firefox 3.1

"There were a number of features that we held back from Firefox 3 because they weren't quite ready - but they were nearly complete,"  Mike Shroepfer  VP engineering at Mozilla wrote in a mailing list entry

The features that Shroepfer includes are some additional interface integrations, performance tuning and JSON DOM binding to name a few. Certainly these are things that will improve Firefox, but in my simple view none of the things that got left out of Firefox 3 are as dramatic as what got left out of Firefox 2.  The omission of the Places (new bookmark/history engine) was a major feature drop in my view , and I'm thrilled that it's fully and properly implemented in Firefox 3.

So sure Firefox 3.1 will be interesting...but the bigger jump won't come till 2009 (or beyond) with Firefox 4, which will be based on the new Mozilla2 platform that Mozilla CTO Brendan Eich has been talking about for some time. Firefox 4 will likely be the first browser to herald the most important evolution of JavaScript since Eich created it over a decade ago.