I'm not using it for long, but it seems to work very well, it also helps in writing not only in fixing grammar errors, but also warns you about duplicate words and other little stuff (I guess the pro version is even better).
Not only it does work well, but is also very pleasant to look at, both the configuration page and the little pop up when you want to fix the words etc... I love the style!
Sadly it does not work in every fields, like for example Google search bar or the one where I'm writing the review! This is a little bit ironic
Edit to answer: I didn't know about mozilla limiting them on the site, but I think if it could be possible to have the spellcheck even for those text field (maybe with a settings to disable them?).