This is indeed a very useful extension, and I have gotten it programmed "into my fingers". Unfortunately, it became less useful with the most recent release of Thunderbird. Now the popup window with a selection of recent relevant folders doesn't happen anymore, which is a real shame. Developers, please bring back this feature!
Also, the automatic save folder name, which appears in the status bar at the bottom, is very frequently incorrect, being just a folder that I save to quite often, but not relevant to the one I want to save to; yet I have done enough saves to that one that Nostalgy should be "trained" on it.