It does have a spell checker built in but I prefer PPF’s SpellBound integration. SpellBound gives spell check as you type. The Firefox integration is a bit buggy. I could not get it to open as a sidebar. It opens in a new window but I care much about it.
One major bug is in the ‘Send to Deepest Sender’ menu option. It eats up part of the post I have already typed in. 
I am typing this post for the third time since I tried to post the features list from their website. So you have to go to their site to check out the features. 
Plus the undo option seems to be screwed up.
Otherwise it looks like a nice competitor to PFF.
Update: Hmmm… Looks like it does not spell check the title. Fixed a typo there.
Update 2: It also stripped out all the paragraph tags! I just put them back. Hey this is not ready for prime time I guess.