I must say I am a bit impressed by the LanguageTool plugin.
It seems to work quite well (running a quote of yours through it ;-)
It also gets generally favourable reviews.
The only drawback I can see using it is that it is probably a little bit overkill for the purpose of posting forum messages, resulting in lots of suggestions that are probably justified but not that important for a forum post.
I'm going to leave it on for a while, and see if it will start to bother me at a certain moment.
I noticed there is also some Firefox dictionary repository:https://addons.thunderbird.net/en-us/firefox/language-tools/
But those files could well be identical to what you get when you customise the preferences settings in FF.
What seems a bit annoying about LanguageTools is that it still alerts me on something like 'organise'.
I have set LT to En-En, and when clicking on 'organise' it suggests organize, but also says that organise is not wrong.
I don't need alerts like that…