For those of you who don’t keep a miniature English teacher on your desktop to help you with your writing woes, there is After the Deadline. Now, I have already told you before about the great bookmarklet they provide to help you with your spelling and grammar errors, but now they have a Firefox extension too.
After the Deadline bring a smart, contextual spelling and grammar checker to the Firefox browser. Once installed, all you have to do is click on the check icon displayed on text areas of web forms to check your grammar and your spelling. Click it one more time to finish your proofreading.
You can see it in action here:
I just can not highly suggest using this add-on for Firefox enough. You can pick up the After the Deadline Firefox add-on on the Firefox Add-ons web site.