Grammarly Lite bills itself as the smart spellchecker that allows you to write error-free anywhere on the web. This Firefox add-on llooked good everywhere we could see a text field, but it didn't play nice with everything. Facebook posts and blog comments turned out well, but proprietary tools and Gmail, for example, weren't so successful.
Where it didn't play nice, Grammarly had problems picking up basic mistakes, and completely failed to notice some deliberate contextual howlers. Give Grammarly Lite a go by all means, but give it a test run (including some deliberate mistakes) before you rely on it for real.