This is a test of the Deepest Sender extension for Firefox. It seems fairly polished. Nice. It even sports color :-)

The only UI problems I see are the lack of labels on the various selects over on the left of the standalone window, and the double toolbar for formating when one would suffice (again, in the standalone window).  With the LJ user photo and user select  moved up to the top of that column's available space (a prefrence option), there's room for labels on the selects below that. Also, the window size is a bit small. Even for medium resolution screens, it could have an editor window that's about twice as wide. Over half the internet population is using 1020 or greater resolution screens.

The sidebar option for Firefox is very nice. I'd recommend dropping the "Deepest Sender" toolbar which would put the real menu in its rightful place.  I see that in the sidebar it's somewhat necessary to have the toolbar broken into two rows. Also, in the sidebar, the photo and user select could be moved to the left and there might be room for the various select widgets seen in the full window.

Finally, we need an icon that fits in the default Firefox theme's available toolbar space (23x23, not the current 30x31). I opened up the extension XPI and hacked one in that fits a bit better, but it'd be nice for someone with original art to make something a bit crisper. It might even be worth going one pixel shorter on the bottom.

I also noticed a problem with cursor placement when moving between the different view tabs. That might be a Gecko bug and not DS's problem.

Here's what it looks like in the Firefox sidebar:

And here's what it looks like as a standalone window:

Now, where's the extensions for Movable Type and Blogger? Surely someone's working on that :-)