This article explains how to enable inline spell checking so that mistakes are corrected immediately as you type in Mozilla Thunderbird.
How to add a spell checker to your Thunderbird
Use Mozilla Thunderbird's spell checker to find and correct typos and spelling errors. Inline spell checking lets you correct errors as you type. To have Mozilla Thunderbird check the spelling in emails as you write them:
Go to the Thunderbird menu and select Preferences.
Select the Composition tab.
