Mozilla Thunderbird is not able to use Microsoft Word spelling engine, but checking spelling is still possible.
Many of us use Mozilla Thunderbird for emailing. It is an ancient but powerful free email client. Spelling errors in emails you send look ugly. So you should know how to check spelling in Thunderbird.
First, you need to perform some preparations in your Thunderbird so it could check spelling. Install glossaries for languages you use. To do this, select Tools > Add-ons:
Add-ons Manager tab opens. You can see the list of installed dictionaries. Add/remove dictionaries you want. Please note that there can be several different dictionaries for a single language; choose the one you want.
Now spelling check becomes possible. To run it in a email you type, select Options > Check spelling... or press CTRL+SHIFT+P or, press well-known and familiar F7 key that is used by default for starting spelling check in the overwhelming majority of programs and applications intended for text processing.
The familiar dialog box appears:
Select the correct language here. Then, you can process the text of your email in a way you do this in Word.