Brilliant Rated 5 out of 5 stars
I can't remember when I first started using this but have never had any problems. I use it in Thunderbird and Firefox
Came to this page to add it again after an uninstall and reinstall of Firefox.
I was surprised to see a reviewer say it didn't recognise 'colour'. It does, as well as 'centre' and other British English words, but you have to select the language/dictionary to be used when you install the addon. (right click context menu), as stated by a reviewer about 5 or 6 reviews down. After that it works fine.