Excellent but one bug and one Feature Request Rated 5 out of 5 stars
First, thanks for this extension! I still can't believe the devs removed this basic functionality from the core code.
Ok, the bug, minor as it is:
When you have Newsgroups, it shows 3 ?'s (???) under the 'Size' column next to each newsgroup. I'd suggest it should just be blank, or more properly, reflect the size being taken up ON DISK for the local cache for that newsgroup.
Now, for the Feature Request (sorry it is so lengthy):
There is a feature available in the Mailtweak 1.0pre10 addon (http://mailtweak.mozdev.org/installation.html) that I absolutely love...
I have a little video of my minimalist UI's for both Firefox and Thunderbird showing this in action here:
It provides the ability to hide the email folder pane (and the email header pane (and the dev for the CompactHeader extension has already expressed interest in adding this capability to it for auto show/hiding the email headers), and auto-show it simply by mousing over the left side of the window (where the folder pane would normally be). When your mouse gets right up to the left edge, it pops out to the right, and when you move the mouse out of the folder pane, it disappears again. This provides a huge increase in usable screen real estate, and is especially useful on netbooks. It would be nice if the 'hotspot' (in pixels) could be a config option.
One thing though... it would be great if it could be implemented in a (semi-transparent) Panel, the way the auto show/hide of the Tab bar in Firefox (provided by the Treestyle Tab addon) works in the above video - meaning, it doesn't SHIFT the message list/detail panes when the folder pane is displayed, it 'overlays' them until you mouse out of the folder pane, then it disappears again...