If only. Grammar checkers would have to be very, very comprehensive. And if it were to be a Firefox add-on, it would probably suck most of the time. Allow me to demonstrate:
Nevermind.
You, see, Firefox spell checker underlined that. Need more?
Teleport.
Not a word? Really?
I'm sure a grammar checker would underline the following:
A new DS game would be most enjoyable!
Yeah.