Please add Undo (and Redo) support to tab manipulation. Sometimes I mess-up my carefully positioned layout by accident. Same goes for closing individual tabs and groups of tabs - this should be reversible action. Currently you can restore closed tabs with Ctrl+Shift+T, but they loose the grouping and you are forced to leave the TabCandy view. What I mean is for Undo/Redo to work directly in TC view.
@Ethan: "I don't know if supporting undo of layout changes has a lot of value for more users."
I would hope it does. The thing is, it would be a good design and given the Mozilla's own Aza Raskin is well aware of it (see http://www.alistapart.com/articles/neveruseawarning) I'm hoping we'll see that kind of sensible design throughout Firefox in not that distant future.
Granted, I'm very glad Firefox already enforces these principles to a point but there's certainly place to enforce it even further.
Ethan Sisson commented
There has been talk of a universal Firefox undo. Whatever your last action was, whether editing text, closing a set of tabs in Tab Candy, messing up your tab/sets layout (possibly), the undo button/hot key would reverse it. I don't know if supporting undo of layout changes has a lot of value for more users.
Kevin Suttle commented
I would take this a step further, adding CTRL/CMD-Z to Undo Closed Tab (a la Safari), even outside of Tab Candy.