It's a bit slow/choppy for me in Firefox, but everything works fine. I really like your single-click and double-click implementation. It makes playing the game very streamlined.
One feature I really like in my iOS version of Solitaire, compared to the Windows 7 one for example, is that there's a feature to only include games that can actually be won. With Windows 7 solitaire, 80% of the games seem to give me the "There are no moves left" message within 30 seconds and it's just annoying that it happens so often.
I'd also like to see a timer and possibly a scoring implementation, but the timer is more important to me personally.
The canvas integration in FireFox is strangely slow, even IE is about 5x faster.
Scoring and hardness (only winnable games) has been added to the 1.1 feature list.
Likewise I will look into how complex it actually is to warn the user that there is no more valid moves.