I have been remiss in not posting a status update about this in two weeks, but that is mostly because we have unfortunately had to slow down work on this feature. The problem is that a string freeze became necessary for all toolkit code (the code shared with the Firefox mobile browser and where this feature would have lived). Unfortunately this all came up over a small period when I was travelling long distances and having to take time out to satisfy immigration authorities that I wasn’t a terrorist intent on bringing down the U.S. government.
In the end there just wasn’t enough time to finalise the UI and be confident that it was correct before the freeze. This means that this feature isn’t going to make Firefox 3.6, however the intention is to continue working on it to get it into Firefox 3.7. It will just be on hold for a short time since I need to devote myself to Firefox 3.6 blockers.