https://support.mozilla.org/en-US/products/thunderbird/learn-basics-get-started

I don't know of any training courses for Thunderbird. I just looked on Google and found one for which you need to register. That allows you to "play" the first three sessions for free, after which you need to pay. I have no idea what this course teaches nor at what level. It appeared to leapfrog over the basics and go into encryption and the generation of keys. Despite being named "quickstart" it seemed to go on beyond elementary topics very quickly. It's dated 2010 and so, like much material on the 'net, is out of date. In general this doesn't matter, but it may get you confused over small detail changes. But I most definitely couldn't (and wouldn't) recommend it without trying it or hearing a trustworthy report on it. I think many reading your query would be a little confused. Thunderbird is an email program. It uses conventional menus. It would be viewed as many as intuitive or self-explanatory, and it's hard to guess at what routine aspects of it you might request training in. There is a manual here: http://write.flossmanuals.net/thunderbird/about-thunderbird/ but again it is somewhat out of date. So, what is your starting point? Do you use email already? Do you use a client program such as Outlook, or do you do it via a web browser? What do you want from a training course? Are there any particular words or phrases related to email or Thunderbird that you have encountered that you don't understand? Has anything I have written here made sense to you? ;-)

Go to YouTube and search Using Thunderbird Email.