Guide on how to set only one French dictionary Rated 5 out of 5 stars
First, close Thunderbird and navigate to your profile, which is usually placed under “C:\Program Files\Mozilla Thunderbird\”.
In your profile, you will see a folder named: “extensions”. Open it. You should see another folder named: “email@example.com”. Go in folder named: “dictionaries”. In that folder, you should see the four different versions of the French dictionaries. The dictionary files come in pairs with extensions “.aff” and “.dic”. Simply remove the pairs you do not want to use and place them in a safe folder somewhere on your desktop, or anywhere else, just as a backup copy. It is important that you only keep in there the dictionary that you want to use with Thunderbird (that is composed of the pair of files with extensions “.aff” and “.dic”). Once this is done, you can launch Thunderbird. The only dictionary that should be available is the one you have chosen... Enjoy!