Rated 4 out of 5 stars

Works very well.
But I would suggest the ability to disable the checker per account in the settings. For example, I have three email accounts. Two are in English and LT works fine for them. The third one is Slovak where I write quickly, without diacritics. LT cannot handle it, it marks almost everything with red. It would be great if I could disable this account in LT settings. I would use standard Thunderbird's dictionary sk-sk-ascii for it. Even no spell checker is better than a wrong one.

This review is for a previous version of the add-on (6.0.5).