The big failing of Firefox is the lack of correction as you type. This addresses it 100% for me. Thank you, Marco!

There are some words missing, but let's acclaim the author's big work for making it!

Thank you, OMG the default spell check is crappy!

Non-native speaker communicating with other non-native speakers in British English, simply because I hate American English.
Works really well! Thank you.

This is one of the best dictionaries available on any platform/app/system. Not only has it the most complete set of words, venturing into many different branches of knowledge, but it's also very actively updated, with lots of words added and revised every month. If it's taken into account that this is an open-source initiative, it's not only amazing, it's really amusing. I wish dictionaries for other languages were this well maintained!