Two small suggestions:
* When in the two window mode, make ctrl+s do an instant refresh on the preview, and preventDefault on the event so that it doesn't open up the browser save window. I save constantly out of habit, so that gets distracting.
* For the 'inspect element' control: Don't use borders on the active element, because it causes the layout to jump around and if you already have a border applied to an element, it wipes it out. The easiest way to fix this would be to use 'outline' instead - and if you apply it with a class instead of inline styles, it will not wipe out existing outlines after leaving (as it does with borders now). If you really want to make it work to not replace existing outlines, see the 'Firebug.Inspector.BoxModelHighlighter' in http://code.google.com/p/fbug/source/browse/branches/firebug... for an implementation that uses completely different elements instead of changing the style of the active element.